gup: document and work around "COW can break either way" issue

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Doing a "get_user_pages()" on a copy-on-write page for reading can be
ambiguous: the page can be COW'ed at any time afterwards, and the direction of a COW event isn't defined.

Yes, whoever writes to it will generally do the COW, but if the thread that did the get_user_pages() unmapped the page before the write (and that could happen due to memory pressure in addition to any outright action), the writer could also just take over the old page instead.

End result: the get_user_pages() call might result in a page pointer that is no longer associated with the original VM, and is associated with - and controlled by - another VM having taken it over instead.

So when doing a get_user_pages() on a COW mapping, the only really safe thing to do would be to break the COW when getting the page, even when only getting it for reading.

At the same time, some users simply don't even care.

For example, the perf code wants to look up the page not because it cares about the page, but because the code simply wants to look up the physical address of the access for informational purposes, and doesn't really care about races when a page might be unmapped and remapped elsewhere.

This adds logic to force a COW event by setting FOLL_WRITE on any copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page pointer as a result.

The current semantics end up being:

- __get_user_pages_fast(): no change. If you don't ask for a write, you won't break COW. You'd better know what you're doing.

- get_user_pages_fast(): the fast-case "look it up in the page tables without anything getting mmap_sem" now refuses to follow a read-only page, since it might need COW breaking. Which happens in the slow path - the fast path doesn't know if the memory might be COW or not.

- get_user_pages() (including the slow-path fallback for gup_fast()):
for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with very similar semantics to FOLL_FORCE.

If it turns out that we want finer granularity (ie "only break COW when it might actually matter" - things like the zero page are special and don't need to be broken) we might need to push these semantics deeper into the lookup fault path. So if people care enough, it's possible that we might end up adding a new internal FOLL_BREAK_COW flag to go with the internal FOLL_COW flag we already have for tracking "I had a COW".

Alternatively, if it turns out that different callers might want to explicitly control the forced COW break behavior, we might even want to make such a flag visible to the users of get_user_pages() instead of using the above default semantics.

But for now, this is mostly commentary on the issue (this commit message being a lot bigger than the patch, and that patch in turn is almost all comments), with that minimal "enable COW breaking early" logic using the existing FOLL_WRITE behavior.

[ It might be worth noting that we've always had this ambiguity, and it
could arguably be seen as a user-space issue.

You only get private COW mappings that could break either way in situations where user space is doing cooperative things (ie fork() before an execve() etc), but it _is_ surprising and very subtle, and fork() is supposed to give you independent address spaces.

So let's treat this as a kernel issue and make the semantics of get_user_pages() easier to understand. Note that obviously a true shared mapping will still get a page that can change under us, so this does _not_ mean that get_user_pages() somehow returns any "stable" page ]

[surenb: backport notes
Since gup_pgd_range does not exist, made appropriate changes on the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead. Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd, gup_huge_pd and gup_pud_range.
Removed FOLL_PIN usage in should_force_cow_break since it's missing in the earlier kernels.]

Reported-by Jann Horn <[email protected]>

Tested-by Christoph Hellwig <[email protected]>

Acked-by Oleg Nesterov <[email protected]>

Acked-by Kirill Shutemov <[email protected]>

Acked-by Jan Kara <[email protected]>

Cc Andrea Arcangeli <[email protected]>

Cc Matthew Wilcox <[email protected]>

Signed-off-by Linus Torvalds <[email protected]>

[surenb: backport to 4.4 kernel]

Cc [email protected] # 4.4.x

Signed-off-by Suren Baghdasaryan <[email protected]>

---
 mm/gup.c         | 48 ++++++++++++++++++++++++++++++++++++++++--------
 mm/huge_memory.c |  7 +++----
 2 files changed, 43 insertions(+), 12 deletions(-)

mm/gup.c +40 -8

index 4c5857889e9d..c80cdc408228 100644
@@ -59,13 +59,22 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
 }
 
 /*
- * FOLL_FORCE can write to even unwritable pte's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
+ * but only after we've gone through a COW cycle and they are dirty.
  */
 static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
 {
-       return pte_write(pte) ||
-               ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+       return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
+}
+
+/*
+ * A (separate) COW fault might break the page the other way and
+ * get_user_pages() would return the page from what is now the wrong
+ * VM. So we need to force a COW break at GUP time even for reads.
+ */
+static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
+{
+       return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,

@@ -509,12 +518,18 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                        if (!vma || check_vma_flags(vma, gup_flags))
                                return i ? : -EFAULT;
                        if (is_vm_hugetlb_page(vma)) {
+                               if (should_force_cow_break(vma, foll_flags))
+                                       foll_flags |= FOLL_WRITE;
                                i = follow_hugetlb_page(mm, vma, pages, vmas,
                                                &start, &nr_pages, i,
-                                               gup_flags);
+                                               foll_flags);
                                continue;
                        }
                }
+
+               if (should_force_cow_break(vma, foll_flags))
+                       foll_flags |= FOLL_WRITE;
+
 retry:
                /*
                 * If we have a pending SIGKILL, don't keep faulting pages and

@@ -1346,6 +1361,10 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 /*
  * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
  * the regular GUP. It will only return non-negative values.
+ *
+ * Careful, careful! COW breaking can go either way, so a non-write
+ * access can get ambiguous page results. If you call this function without
+ * 'write' set, you'd better be sure that you're ok with that ambiguity.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
                          struct page **pages)

@@ -1375,6 +1394,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
         *
         * We do not adopt an rcu_read_lock(.) here as we also want to
         * block IPIs that come from THPs splitting.
+        *
+        * NOTE! We allow read-only gup_fast() here, but you'd better be
+        * careful about possible COW pages. You'll get _a_ COW page, but
+        * not necessarily the one you intended to get depending on what
+        * COW event happens after this. COW may break the page copy in a
+        * random direction.
         */
 
        local_irq_save(flags);

@@ -1385,15 +1410,22 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
                next = pgd_addr_end(addr, end);
                if (pgd_none(pgd))
                        break;
+               /*
+                * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+                * because get_user_pages() may need to cause an early COW in
+                * order to avoid confusing the normal COW routines. So only
+                * targets that are already writable are safe to do by just
+                * looking at the page tables.
+                */
                if (unlikely(pgd_huge(pgd))) {
-                       if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
+                       if (!gup_huge_pgd(pgd, pgdp, addr, next, 1,
                                          pages, &nr))
                                break;
                } else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
                        if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
-                                        PGDIR_SHIFT, next, write, pages, &nr))
+                                        PGDIR_SHIFT, next, 1, pages, &nr))
                                break;
-               } else if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+               } else if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
                        break;
        } while (pgdp++, addr = next, addr != end);
        local_irq_restore(flags);

mm/huge_memory.c +3 -5

index 6404e4fcb4ed..fae45c56e2ee 100644
@@ -1268,13 +1268,12 @@ out_unlock:
 }
 
 /*
- * FOLL_FORCE can write to even unwritable pmd's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
+ * but only after we've gone through a COW cycle and they are dirty.
  */
 static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
 {
-       return pmd_write(pmd) ||
-              ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+       return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
--

2.33.0.882.g93a45727a2-goog

Quoted 4 lines · Suren Baghdasaryan

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

Sorry Greg, I must have screwed up something and this "From:" line appeared twice. Do you want me to resent these backports or you can simply drop the duplicate line?

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Doing a "get_user_pages()" on a copy-on-write page for reading can be
ambiguous: the page can be COW'ed at any time afterwards, and the direction of a COW event isn't defined.

Yes, whoever writes to it will generally do the COW, but if the thread that did the get_user_pages() unmapped the page before the write (and that could happen due to memory pressure in addition to any outright action), the writer could also just take over the old page instead.

End result: the get_user_pages() call might result in a page pointer that is no longer associated with the original VM, and is associated with - and controlled by - another VM having taken it over instead.

So when doing a get_user_pages() on a COW mapping, the only really safe thing to do would be to break the COW when getting the page, even when only getting it for reading.

At the same time, some users simply don't even care.

For example, the perf code wants to look up the page not because it cares about the page, but because the code simply wants to look up the physical address of the access for informational purposes, and doesn't really care about races when a page might be unmapped and remapped elsewhere.

This adds logic to force a COW event by setting FOLL_WRITE on any copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page pointer as a result.

The current semantics end up being:

- __get_user_pages_fast(): no change. If you don't ask for a write, you won't break COW. You'd better know what you're doing.

- get_user_pages_fast(): the fast-case "look it up in the page tables without anything getting mmap_sem" now refuses to follow a read-only page, since it might need COW breaking. Which happens in the slow path - the fast path doesn't know if the memory might be COW or not.

- get_user_pages() (including the slow-path fallback for gup_fast()):
for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with very similar semantics to FOLL_FORCE.

If it turns out that we want finer granularity (ie "only break COW when it might actually matter" - things like the zero page are special and don't need to be broken) we might need to push these semantics deeper into the lookup fault path. So if people care enough, it's possible that we might end up adding a new internal FOLL_BREAK_COW flag to go with the internal FOLL_COW flag we already have for tracking "I had a COW".

Alternatively, if it turns out that different callers might want to explicitly control the forced COW break behavior, we might even want to make such a flag visible to the users of get_user_pages() instead of using the above default semantics.

But for now, this is mostly commentary on the issue (this commit message being a lot bigger than the patch, and that patch in turn is almost all comments), with that minimal "enable COW breaking early" logic using the existing FOLL_WRITE behavior.

[ It might be worth noting that we've always had this ambiguity, and it
could arguably be seen as a user-space issue.

You only get private COW mappings that could break either way in situations where user space is doing cooperative things (ie fork() before an execve() etc), but it _is_ surprising and very subtle, and fork() is supposed to give you independent address spaces.

So let's treat this as a kernel issue and make the semantics of get_user_pages() easier to understand. Note that obviously a true shared mapping will still get a page that can change under us, so this does _not_ mean that get_user_pages() somehow returns any "stable" page ]

[surenb: backport notes
Since gup_pgd_range does not exist, made appropriate changes on the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead. Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd, gup_huge_pd and gup_pud_range.
Removed FOLL_PIN usage in should_force_cow_break since it's missing in the earlier kernels.]

Reported-by Jann Horn <[email protected]>

Tested-by Christoph Hellwig <[email protected]>

Acked-by Oleg Nesterov <[email protected]>

Acked-by Kirill Shutemov <[email protected]>

Acked-by Jan Kara <[email protected]>

Cc Andrea Arcangeli <[email protected]>

Cc Matthew Wilcox <[email protected]>

Signed-off-by Linus Torvalds <[email protected]>

[surenb: backport to 4.4 kernel]

Cc [email protected] # 4.4.x

Signed-off-by Suren Baghdasaryan <[email protected]>

---
 mm/gup.c         | 48 ++++++++++++++++++++++++++++++++++++++++--------
 mm/huge_memory.c |  7 +++----
 2 files changed, 43 insertions(+), 12 deletions(-)

mm/gup.c +40 -8

index 4c5857889e9d..c80cdc408228 100644
@@ -59,13 +59,22 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
 }
 
 /*
- * FOLL_FORCE can write to even unwritable pte's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
+ * but only after we've gone through a COW cycle and they are dirty.
  */
 static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
 {
-	return pte_write(pte) ||
-		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
+}
+
+/*
+ * A (separate) COW fault might break the page the other way and
+ * get_user_pages() would return the page from what is now the wrong
+ * VM. So we need to force a COW break at GUP time even for reads.
+ */
+static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
+{
+	return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,

@@ -509,12 +518,18 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 			if (!vma || check_vma_flags(vma, gup_flags))
 				return i ? : -EFAULT;
 			if (is_vm_hugetlb_page(vma)) {
+				if (should_force_cow_break(vma, foll_flags))
+					foll_flags |= FOLL_WRITE;
 				i = follow_hugetlb_page(mm, vma, pages, vmas,
 						&start, &nr_pages, i,
-						gup_flags);
+						foll_flags);
 				continue;
 			}
 		}
+
+		if (should_force_cow_break(vma, foll_flags))
+			foll_flags |= FOLL_WRITE;
+
 retry:
 		/*
 		 * If we have a pending SIGKILL, don't keep faulting pages and

@@ -1346,6 +1361,10 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 /*
  * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
  * the regular GUP. It will only return non-negative values.
+ *
+ * Careful, careful! COW breaking can go either way, so a non-write
+ * access can get ambiguous page results. If you call this function without
+ * 'write' set, you'd better be sure that you're ok with that ambiguity.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)

@@ -1375,6 +1394,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	 *
 	 * We do not adopt an rcu_read_lock(.) here as we also want to
 	 * block IPIs that come from THPs splitting.
+	 *
+	 * NOTE! We allow read-only gup_fast() here, but you'd better be
+	 * careful about possible COW pages. You'll get _a_ COW page, but
+	 * not necessarily the one you intended to get depending on what
+	 * COW event happens after this. COW may break the page copy in a
+	 * random direction.
 	 */
 
 	local_irq_save(flags);

@@ -1385,15 +1410,22 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 		next = pgd_addr_end(addr, end);
 		if (pgd_none(pgd))
 			break;
+		/*
+		 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+		 * because get_user_pages() may need to cause an early COW in
+		 * order to avoid confusing the normal COW routines. So only
+		 * targets that are already writable are safe to do by just
+		 * looking at the page tables.
+		 */
 		if (unlikely(pgd_huge(pgd))) {
-			if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
+			if (!gup_huge_pgd(pgd, pgdp, addr, next, 1,
 					  pages, &nr))
 				break;
 		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
 			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
-					 PGDIR_SHIFT, next, write, pages, &nr))
+					 PGDIR_SHIFT, next, 1, pages, &nr))
 				break;
-		} else if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+		} else if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
 			break;
 	} while (pgdp++, addr = next, addr != end);
 	local_irq_restore(flags);

mm/huge_memory.c +3 -4

index 6404e4fcb4ed..fae45c56e2ee 100644
@@ -1268,13 +1268,12 @@ out_unlock:
 }
 
 /*
- * FOLL_FORCE can write to even unwritable pmd's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
+ * but only after we've gone through a COW cycle and they are dirty.
  */
 static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
 {
-	return pmd_write(pmd) ||
-	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+	return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,

Signature · 1 line

2.33.0.882.g93a45727a2-goog

Quoted 81 lines · Suren Baghdasaryan

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Doing a "get_user_pages()" on a copy-on-write page for reading can be
ambiguous: the page can be COW'ed at any time afterwards, and the direction of a COW event isn't defined.

Yes, whoever writes to it will generally do the COW, but if the thread that did the get_user_pages() unmapped the page before the write (and that could happen due to memory pressure in addition to any outright action), the writer could also just take over the old page instead.

End result: the get_user_pages() call might result in a page pointer that is no longer associated with the original VM, and is associated with - and controlled by - another VM having taken it over instead.

So when doing a get_user_pages() on a COW mapping, the only really safe thing to do would be to break the COW when getting the page, even when only getting it for reading.

At the same time, some users simply don't even care.

For example, the perf code wants to look up the page not because it cares about the page, but because the code simply wants to look up the physical address of the access for informational purposes, and doesn't really care about races when a page might be unmapped and remapped elsewhere.

This adds logic to force a COW event by setting FOLL_WRITE on any copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page pointer as a result.

The current semantics end up being:

- __get_user_pages_fast(): no change. If you don't ask for a write, you won't break COW. You'd better know what you're doing.

- get_user_pages_fast(): the fast-case "look it up in the page tables without anything getting mmap_sem" now refuses to follow a read-only page, since it might need COW breaking. Which happens in the slow path - the fast path doesn't know if the memory might be COW or not.

- get_user_pages() (including the slow-path fallback for gup_fast()):
for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with very similar semantics to FOLL_FORCE.

If it turns out that we want finer granularity (ie "only break COW when it might actually matter" - things like the zero page are special and don't need to be broken) we might need to push these semantics deeper into the lookup fault path. So if people care enough, it's possible that we might end up adding a new internal FOLL_BREAK_COW flag to go with the internal FOLL_COW flag we already have for tracking "I had a COW".

Alternatively, if it turns out that different callers might want to explicitly control the forced COW break behavior, we might even want to make such a flag visible to the users of get_user_pages() instead of using the above default semantics.

But for now, this is mostly commentary on the issue (this commit message being a lot bigger than the patch, and that patch in turn is almost all comments), with that minimal "enable COW breaking early" logic using the existing FOLL_WRITE behavior.

[ It might be worth noting that we've always had this ambiguity, and it
could arguably be seen as a user-space issue.

You only get private COW mappings that could break either way in situations where user space is doing cooperative things (ie fork() before an execve() etc), but it _is_ surprising and very subtle, and fork() is supposed to give you independent address spaces.

So let's treat this as a kernel issue and make the semantics of get_user_pages() easier to understand. Note that obviously a true shared mapping will still get a page that can change under us, so this does _not_ mean that get_user_pages() somehow returns any "stable" page ]

Quoted 7 lines

[surenb: backport notes
Since gup_pgd_range does not exist, made appropriate changes on the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead. Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd, gup_huge_pd and gup_pud_range.
Removed FOLL_PIN usage in should_force_cow_break since it's missing in the earlier kernels.]

I'd be really careful with backporting this to stable. There was a lot of userspace breakage caused by this change if I remember right which needed to be fixed up later. There is a nice summary at
https://lwn.net/Articles/849638/ and https://lwn.net/Articles/849876/ and some problems are still being found...

Quoted 90 lines · Jan Kara

Quoted 81 lines · Suren Baghdasaryan

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Doing a "get_user_pages()" on a copy-on-write page for reading can be
ambiguous: the page can be COW'ed at any time afterwards, and the direction of a COW event isn't defined.

Yes, whoever writes to it will generally do the COW, but if the thread that did the get_user_pages() unmapped the page before the write (and that could happen due to memory pressure in addition to any outright action), the writer could also just take over the old page instead.

End result: the get_user_pages() call might result in a page pointer that is no longer associated with the original VM, and is associated with - and controlled by - another VM having taken it over instead.

So when doing a get_user_pages() on a COW mapping, the only really safe thing to do would be to break the COW when getting the page, even when only getting it for reading.

At the same time, some users simply don't even care.

For example, the perf code wants to look up the page not because it cares about the page, but because the code simply wants to look up the physical address of the access for informational purposes, and doesn't really care about races when a page might be unmapped and remapped elsewhere.

This adds logic to force a COW event by setting FOLL_WRITE on any copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page pointer as a result.

The current semantics end up being:

- __get_user_pages_fast(): no change. If you don't ask for a write, you won't break COW. You'd better know what you're doing.

- get_user_pages_fast(): the fast-case "look it up in the page tables without anything getting mmap_sem" now refuses to follow a read-only page, since it might need COW breaking. Which happens in the slow path - the fast path doesn't know if the memory might be COW or not.

- get_user_pages() (including the slow-path fallback for gup_fast()):
for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with very similar semantics to FOLL_FORCE.

If it turns out that we want finer granularity (ie "only break COW when it might actually matter" - things like the zero page are special and don't need to be broken) we might need to push these semantics deeper into the lookup fault path. So if people care enough, it's possible that we might end up adding a new internal FOLL_BREAK_COW flag to go with the internal FOLL_COW flag we already have for tracking "I had a COW".

Alternatively, if it turns out that different callers might want to explicitly control the forced COW break behavior, we might even want to make such a flag visible to the users of get_user_pages() instead of using the above default semantics.

But for now, this is mostly commentary on the issue (this commit message being a lot bigger than the patch, and that patch in turn is almost all comments), with that minimal "enable COW breaking early" logic using the existing FOLL_WRITE behavior.

[ It might be worth noting that we've always had this ambiguity, and it
could arguably be seen as a user-space issue.

You only get private COW mappings that could break either way in situations where user space is doing cooperative things (ie fork() before an execve() etc), but it _is_ surprising and very subtle, and fork() is supposed to give you independent address spaces.

So let's treat this as a kernel issue and make the semantics of get_user_pages() easier to understand. Note that obviously a true shared mapping will still get a page that can change under us, so this does _not_ mean that get_user_pages() somehow returns any "stable" page ]

Quoted 7 lines

[surenb: backport notes
Since gup_pgd_range does not exist, made appropriate changes on the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead. Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd, gup_huge_pd and gup_pud_range.
Removed FOLL_PIN usage in should_force_cow_break since it's missing in the earlier kernels.]

Quoted 5 lines

I'd be really careful with backporting this to stable. There was a lot of userspace breakage caused by this change if I remember right which needed to be fixed up later. There is a nice summary at
https://lwn.net/Articles/849638/ and https://lwn.net/Articles/849876/ and some problems are still being found...

Yeah that was my initial reaction. But looks like back in April we agreed that backporting only this commit could be feasible - the relevant subthread starts around here [1]. The known breakage for just this commit was uffd functionality introduced only in 5.7, and strace on dax on pmem (that was never properly root caused). 5.4 stable already has the backport since year ago, Suren posted 4.14 and 4.19 in April after [1]. Looks like nobody reported issues? Continuing with 4.4 and 4.9 makes this consistent at least, although the risk of breaking something is always there and the CVE probably not worth it, but whatever...

Quoted 97 lines · Vlastimil Babka

Quoted 90 lines · Jan Kara

Quoted 81 lines · Suren Baghdasaryan

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Doing a "get_user_pages()" on a copy-on-write page for reading can be
ambiguous: the page can be COW'ed at any time afterwards, and the direction of a COW event isn't defined.

Yes, whoever writes to it will generally do the COW, but if the thread that did the get_user_pages() unmapped the page before the write (and that could happen due to memory pressure in addition to any outright action), the writer could also just take over the old page instead.

End result: the get_user_pages() call might result in a page pointer that is no longer associated with the original VM, and is associated with - and controlled by - another VM having taken it over instead.

So when doing a get_user_pages() on a COW mapping, the only really safe thing to do would be to break the COW when getting the page, even when only getting it for reading.

At the same time, some users simply don't even care.

For example, the perf code wants to look up the page not because it cares about the page, but because the code simply wants to look up the physical address of the access for informational purposes, and doesn't really care about races when a page might be unmapped and remapped elsewhere.

This adds logic to force a COW event by setting FOLL_WRITE on any copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page pointer as a result.

The current semantics end up being:

- __get_user_pages_fast(): no change. If you don't ask for a write, you won't break COW. You'd better know what you're doing.

- get_user_pages_fast(): the fast-case "look it up in the page tables without anything getting mmap_sem" now refuses to follow a read-only page, since it might need COW breaking. Which happens in the slow path - the fast path doesn't know if the memory might be COW or not.

- get_user_pages() (including the slow-path fallback for gup_fast()):
for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with very similar semantics to FOLL_FORCE.

If it turns out that we want finer granularity (ie "only break COW when it might actually matter" - things like the zero page are special and don't need to be broken) we might need to push these semantics deeper into the lookup fault path. So if people care enough, it's possible that we might end up adding a new internal FOLL_BREAK_COW flag to go with the internal FOLL_COW flag we already have for tracking "I had a COW".

Alternatively, if it turns out that different callers might want to explicitly control the forced COW break behavior, we might even want to make such a flag visible to the users of get_user_pages() instead of using the above default semantics.

But for now, this is mostly commentary on the issue (this commit message being a lot bigger than the patch, and that patch in turn is almost all comments), with that minimal "enable COW breaking early" logic using the existing FOLL_WRITE behavior.

[ It might be worth noting that we've always had this ambiguity, and it
could arguably be seen as a user-space issue.

You only get private COW mappings that could break either way in situations where user space is doing cooperative things (ie fork() before an execve() etc), but it _is_ surprising and very subtle, and fork() is supposed to give you independent address spaces.

So let's treat this as a kernel issue and make the semantics of get_user_pages() easier to understand. Note that obviously a true shared mapping will still get a page that can change under us, so this does _not_ mean that get_user_pages() somehow returns any "stable" page ]

Quoted 7 lines

[surenb: backport notes
Since gup_pgd_range does not exist, made appropriate changes on the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead. Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd, gup_huge_pd and gup_pud_range.
Removed FOLL_PIN usage in should_force_cow_break since it's missing in the earlier kernels.]

Quoted 5 lines

I'd be really careful with backporting this to stable. There was a lot of userspace breakage caused by this change if I remember right which needed to be fixed up later. There is a nice summary at
https://lwn.net/Articles/849638/ and https://lwn.net/Articles/849876/ and some problems are still being found...

Quoted 9 lines

Yeah that was my initial reaction. But looks like back in April we agreed that backporting only this commit could be feasible - the relevant subthread starts around here [1]. The known breakage for just this commit was uffd functionality introduced only in 5.7, and strace on dax on pmem (that was never properly root caused). 5.4 stable already has the backport since year ago, Suren posted 4.14 and 4.19 in April after [1]. Looks like nobody reported issues? Continuing with 4.4 and 4.9 makes this consistent at least, although the risk of breaking something is always there and the CVE probably not worth it, but whatever...

I have had people "complain" that the issue was not fixed on these older kernels, now if that is just because those groups have a "it has a CVE so it must be fixed!" policy or not, it is hard to tell.

But this seems to be exploitable, and we have a reproducer somewhere around here, so it would be nice to get resolved for the reason of it being a bug that we should fix if possible.

So I would err on the side of "lets merge this" as fixing a known issue is ALWAYS better than the fear of "maybe something might break". We can always revert if the latter happens in testing.

thanks,

greg k-h

Quoted 116 lines · Greg KH

Quoted 97 lines · Vlastimil Babka

Quoted 90 lines · Jan Kara

Quoted 81 lines · Suren Baghdasaryan

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Doing a "get_user_pages()" on a copy-on-write page for reading can be
ambiguous: the page can be COW'ed at any time afterwards, and the direction of a COW event isn't defined.

Yes, whoever writes to it will generally do the COW, but if the thread that did the get_user_pages() unmapped the page before the write (and that could happen due to memory pressure in addition to any outright action), the writer could also just take over the old page instead.

End result: the get_user_pages() call might result in a page pointer that is no longer associated with the original VM, and is associated with - and controlled by - another VM having taken it over instead.

So when doing a get_user_pages() on a COW mapping, the only really safe thing to do would be to break the COW when getting the page, even when only getting it for reading.

At the same time, some users simply don't even care.

For example, the perf code wants to look up the page not because it cares about the page, but because the code simply wants to look up the physical address of the access for informational purposes, and doesn't really care about races when a page might be unmapped and remapped elsewhere.

This adds logic to force a COW event by setting FOLL_WRITE on any copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page pointer as a result.

The current semantics end up being:

- __get_user_pages_fast(): no change. If you don't ask for a write, you won't break COW. You'd better know what you're doing.

- get_user_pages_fast(): the fast-case "look it up in the page tables without anything getting mmap_sem" now refuses to follow a read-only page, since it might need COW breaking. Which happens in the slow path - the fast path doesn't know if the memory might be COW or not.

- get_user_pages() (including the slow-path fallback for gup_fast()):
for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with very similar semantics to FOLL_FORCE.

If it turns out that we want finer granularity (ie "only break COW when it might actually matter" - things like the zero page are special and don't need to be broken) we might need to push these semantics deeper into the lookup fault path. So if people care enough, it's possible that we might end up adding a new internal FOLL_BREAK_COW flag to go with the internal FOLL_COW flag we already have for tracking "I had a COW".

Alternatively, if it turns out that different callers might want to explicitly control the forced COW break behavior, we might even want to make such a flag visible to the users of get_user_pages() instead of using the above default semantics.

But for now, this is mostly commentary on the issue (this commit message being a lot bigger than the patch, and that patch in turn is almost all comments), with that minimal "enable COW breaking early" logic using the existing FOLL_WRITE behavior.

[ It might be worth noting that we've always had this ambiguity, and it
could arguably be seen as a user-space issue.

You only get private COW mappings that could break either way in situations where user space is doing cooperative things (ie fork() before an execve() etc), but it _is_ surprising and very subtle, and fork() is supposed to give you independent address spaces.

So let's treat this as a kernel issue and make the semantics of get_user_pages() easier to understand. Note that obviously a true shared mapping will still get a page that can change under us, so this does _not_ mean that get_user_pages() somehow returns any "stable" page ]

Quoted 7 lines

[surenb: backport notes
Since gup_pgd_range does not exist, made appropriate changes on the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead. Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd, gup_huge_pd and gup_pud_range.
Removed FOLL_PIN usage in should_force_cow_break since it's missing in the earlier kernels.]

Quoted 5 lines

I'd be really careful with backporting this to stable. There was a lot of userspace breakage caused by this change if I remember right which needed to be fixed up later. There is a nice summary at
https://lwn.net/Articles/849638/ and https://lwn.net/Articles/849876/ and some problems are still being found...

Quoted 9 lines

Yeah that was my initial reaction. But looks like back in April we agreed that backporting only this commit could be feasible - the relevant subthread starts around here [1]. The known breakage for just this commit was uffd functionality introduced only in 5.7, and strace on dax on pmem (that was never properly root caused). 5.4 stable already has the backport since year ago, Suren posted 4.14 and 4.19 in April after [1]. Looks like nobody reported issues? Continuing with 4.4 and 4.9 makes this consistent at least, although the risk of breaking something is always there and the CVE probably not worth it, but whatever...

I have had people "complain" that the issue was not fixed on these older kernels, now if that is just because those groups have a "it has a CVE so it must be fixed!" policy or not, it is hard to tell.

But this seems to be exploitable, and we have a reproducer somewhere around here, so it would be nice to get resolved for the reason of it being a bug that we should fix if possible.

Quoted 7 lines

So I would err on the side of "lets merge this" as fixing a known issue is ALWAYS better than the fear of "maybe something might break". We can always revert if the latter happens in testing.

thanks,

greg k-h

When we backported this to the Ubuntu kernel based on 4.4, we found a regression that required commit 38e088546522e1e86d2b8f401a1354ad3a9b3303 ("mm: check VMA flags to avoid invalid PROT_NONE NUMA balancing") as a fix.

I tested that this was also the case with the 4.4.y stable-rc tree and I am providing our backport below, which I also tested. The reproducer that regresses reads from /proc/self/mem. Writing to /proc/self/mem seems to have been a bug on 4.4 for a while and is also fixed by this backport, so should be considered in any case.

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Quoted 5 lines · Suren Baghdasaryan

From: Linus Torvalds <[email protected]>

From: Linus Torvalds <[email protected]>

commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.

Both backports now queued up, thanks.

greg k-h

--
To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].

[surenb: backport notes
Since gup_pgd_range does not exist, made appropriate changes on the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead.
Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd,
gup_huge_pd and gup_pud_range.
Removed FOLL_PIN usage in should_force_cow_break since it's missing in the earlier kernels.]

Reported-by Jann Horn <[email protected]>

Tested-by Christoph Hellwig <[email protected]>

Acked-by Oleg Nesterov <[email protected]>

Acked-by Kirill Shutemov <[email protected]>

Acked-by Jan Kara <[email protected]>

Cc Andrea Arcangeli <[email protected]>

Cc Matthew Wilcox <[email protected]>

Signed-off-by Linus Torvalds <[email protected]>

[surenb: backport to 4.4 kernel]

Cc [email protected] # 4.4.x

Signed-off-by Suren Baghdasaryan <[email protected]>

---
  mm/gup.c         | 48 ++++++++++++++++++++++++++++++++++++++++--------
  mm/huge_memory.c |  7 +++----
  2 files changed, 43 insertions(+), 12 deletions(-)

mm/gup.c +40 -8

index 4c5857889e9d..c80cdc408228 100644
@@ -59,13 +59,22 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
  }
 
  /*
- * FOLL_FORCE can write to even unwritable pte's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
+ * but only after we've gone through a COW cycle and they are dirty.
   */
  static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
  {
-	return pte_write(pte) ||
-		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
+}
+
+/*
+ * A (separate) COW fault might break the page the other way and
+ * get_user_pages() would return the page from what is now the wrong
+ * VM. So we need to force a COW break at GUP time even for reads.
+ */
+static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
+{
+	return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
  }
 
  static struct page *follow_page_pte(struct vm_area_struct *vma,

@@ -509,12 +518,18 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
  			if (!vma || check_vma_flags(vma, gup_flags))
  				return i ? : -EFAULT;
  			if (is_vm_hugetlb_page(vma)) {
+				if (should_force_cow_break(vma, foll_flags))
+					foll_flags |= FOLL_WRITE;
  				i = follow_hugetlb_page(mm, vma, pages, vmas,
  						&start, &nr_pages, i,
-						gup_flags);
+						foll_flags);
  				continue;
  			}
  		}
+
+		if (should_force_cow_break(vma, foll_flags))
+			foll_flags |= FOLL_WRITE;
+
  retry:
  		/*
  		 * If we have a pending SIGKILL, don't keep faulting pages and

@@ -1346,6 +1361,10 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
  /*
   * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
   * the regular GUP. It will only return non-negative values.
+ *
+ * Careful, careful! COW breaking can go either way, so a non-write
+ * access can get ambiguous page results. If you call this function without
+ * 'write' set, you'd better be sure that you're ok with that ambiguity.
   */
  int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
  			  struct page **pages)

@@ -1375,6 +1394,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
  	 *
  	 * We do not adopt an rcu_read_lock(.) here as we also want to
  	 * block IPIs that come from THPs splitting.
+	 *
+	 * NOTE! We allow read-only gup_fast() here, but you'd better be
+	 * careful about possible COW pages. You'll get _a_ COW page, but
+	 * not necessarily the one you intended to get depending on what
+	 * COW event happens after this. COW may break the page copy in a
+	 * random direction.
  	 */
 
  	local_irq_save(flags);

@@ -1385,15 +1410,22 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
  		next = pgd_addr_end(addr, end);
  		if (pgd_none(pgd))
  			break;
+		/*
+		 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
+		 * because get_user_pages() may need to cause an early COW in
+		 * order to avoid confusing the normal COW routines. So only
+		 * targets that are already writable are safe to do by just
+		 * looking at the page tables.
+		 */
  		if (unlikely(pgd_huge(pgd))) {
-			if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
+			if (!gup_huge_pgd(pgd, pgdp, addr, next, 1,
  					  pages, &nr))
  				break;
  		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
  			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
-					 PGDIR_SHIFT, next, write, pages, &nr))
+					 PGDIR_SHIFT, next, 1, pages, &nr))
  				break;
-		} else if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+		} else if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
  			break;
  	} while (pgdp++, addr = next, addr != end);
  	local_irq_restore(flags);

mm/huge_memory.c +3 -4

index 6404e4fcb4ed..fae45c56e2ee 100644
@@ -1268,13 +1268,12 @@ out_unlock:
  }
 
  /*
- * FOLL_FORCE can write to even unwritable pmd's, but only
- * after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
+ * but only after we've gone through a COW cycle and they are dirty.
   */
  static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
  {
-	return pmd_write(pmd) ||
-	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+	return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));
  }
 
  struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,