mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma.patch removed from -mm tree

The quilt patch titled
Subject: mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA
has been removed from the -mm tree. Its filename was
mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: David Hildenbrand <[email protected]>
Subject: mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA
Date: Fri, 9 Dec 2022 09:09:12 +0100

Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...).

So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1].

This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE
permissions: primarily pte_modify() and mk_pte().

Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection.

This fixes two known cases:

(a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access.
(b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access.

Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional.

Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general.

Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory in that regard, even though, mixing mprotect with uffd-wp is controversial.

[1] https://lkml.kernel.org/r/[email protected]

---

 fs/userfaultfd.c |   28 ++++++++++++++++++++++------
 mm/mmap.c        |    4 ++++
 2 files changed, 26 insertions(+), 6 deletions(-)

@@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(s
 	return ctx->features & UFFD_FEATURE_INITIALIZED;
 }
 
+static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
+				     vm_flags_t flags)
+{
+	const bool uffd_wp_changed = (vma->vm_flags ^ flags) & VM_UFFD_WP;
+
+	vma->vm_flags = flags;
+	/*
+	 * For shared mappings, we want to enable writenotify while
+	 * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply
+	 * recalculate vma->vm_page_prot whenever userfaultfd-wp changes.
+	 */
+	if ((vma->vm_flags & VM_SHARED) && uffd_wp_changed)
+		vma_set_page_prot(vma);
+}
+
 static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
 				     int wake_flags, void *key)
 {

@@ -618,7 +633,8 @@ static void userfaultfd_event_wait_compl
 		for_each_vma(vmi, vma) {
 			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
 				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
-				vma->vm_flags &= ~__VM_UFFD_FLAGS;
+				userfaultfd_set_vm_flags(vma,
+							 vma->vm_flags & ~__VM_UFFD_FLAGS);
 			}
 		}
 		mmap_write_unlock(mm);

@@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struc
 	octx = vma->vm_userfaultfd_ctx.ctx;
 	if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
-		vma->vm_flags &= ~__VM_UFFD_FLAGS;
+		userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
 		return 0;
 	}
 

@@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_a
 	} else {
 		/* Drop uffd context if remap feature not enabled */
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
-		vma->vm_flags &= ~__VM_UFFD_FLAGS;
+		userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
 	}
 }
 

@@ -895,7 +911,7 @@ static int userfaultfd_release(struct in
 			prev = vma;
 		}
 
-		vma->vm_flags = new_flags;
+		userfaultfd_set_vm_flags(vma, new_flags);
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 	}
 	mmap_write_unlock(mm);

@@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct u
 		 * the next vma was merged into the current one and
 		 * the current one has not been updated yet.
 		 */
-		vma->vm_flags = new_flags;
+		userfaultfd_set_vm_flags(vma, new_flags);
 		vma->vm_userfaultfd_ctx.ctx = ctx;
 
 		if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))

@@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct
 		 * the next vma was merged into the current one and
 		 * the current one has not been updated yet.
 		 */
-		vma->vm_flags = new_flags;
+		userfaultfd_set_vm_flags(vma, new_flags);
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 
 	skip:

@@ -1524,6 +1524,10 @@ int vma_wants_writenotify(struct vm_area
 	if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma))
 		return 1;
 
+	/* Do we need write faults for uffd-wp tracking? */
+	if (userfaultfd_wp(vma))
+		return 1;
+
 	/* Specialty mapping? */
 	if (vm_flags & VM_PFNMAP)
 		return 0;

_

Patches currently in -mm which might be from [email protected] are

mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range.patch mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range-fix.patch mm-mprotect-drop-pgprot_t-parameter-from-change_protection.patch mm-mprotect-drop-pgprot_t-parameter-from-change_protection-fix.patch selftests-vm-cow-add-cow-tests-for-collapsing-of-pte-mapped-anon-thp.patch mm-nommu-factor-out-check-for-nommu-shared-mappings-into-is_nommu_shared_mapping.patch mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch drivers-misc-open-dice-dont-touch-vm_mayshare.patch selftests-mm-define-madv_pageout-to-fix-compilation-issues.patch