mm-drop-the-assumption-that-vm_shared-always-implies-writable.patch removed from -mm tree

merged mm-stable Andrew Morton 1 participant 1 message Oct 18, 2023

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2023-10-18 21:37 UTC #0 lore raw

The quilt patch titled
Subject: mm: drop the assumption that VM_SHARED always implies writable
has been removed from the -mm tree. Its filename was
mm-drop-the-assumption-that-vm_shared-always-implies-writable.patch

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

------------------------------------------------------
From: Lorenzo Stoakes <[email protected]>
Subject: mm: drop the assumption that VM_SHARED always implies writable
Date: Thu, 12 Oct 2023 18:04:28 +0100

Patch series "permit write-sealed memfd read-only shared mappings", v4.

The man page for fcntl() describing memfd file seals states the following about F_SEAL_WRITE:-

Furthermore, trying to create new shared, writable memory-mappings via mmap(2) will also fail with EPERM.

With emphasis on 'writable'. In turns out in fact that currently the kernel simply disallows all new shared memory mappings for a memfd with F_SEAL_WRITE applied, rendering this documentation inaccurate.

This matters because users are therefore unable to obtain a shared mapping to a memfd after write sealing altogether, which limits their usefulness. This was reported in the discussion thread [1] originating from a bug report [2].

This is a product of both using the struct address_space->i_mmap_writable atomic counter to determine whether writing may be permitted, and the kernel adjusting this counter when any VM_SHARED mapping is performed and more generally implicitly assuming VM_SHARED implies writable.

It seems sensible that we should only update this mapping if VM_MAYWRITE is specified, i.e. whether it is possible that this mapping could at any point be written to.

If we do so then all we need to do to permit write seals to function as documented is to clear VM_MAYWRITE when mapping read-only. It turns out this functionality already exists for F_SEAL_FUTURE_WRITE - we can therefore simply adapt this logic to do the same for F_SEAL_WRITE.

We then hit a chicken and egg situation in mmap_region() where the check for VM_MAYWRITE occurs before we are able to clear this flag. To work around this, perform this check after we invoke call_mmap(), with careful consideration of error paths.

Thanks to Andy Lutomirski for the suggestion!

[1]:https://lore.kernel.org/all/[email protected]/ [2]:https://bugzilla.kernel.org/show_bug.cgi?id=217238

This patch (of 3):

There is a general assumption that VMAs with the VM_SHARED flag set are writable. If the VM_MAYWRITE flag is not set, then this is simply not the case.

Update those checks which affect the struct address_space->i_mmap_writable field to explicitly test for this by introducing [vma_]is_shared_maywrite() helper functions.

This remains entirely conservative, as the lack of VM_MAYWRITE guarantees that the VMA cannot be written to.

Link https://lkml.kernel.org/r/[email protected] open here

Link https://lkml.kernel.org/r/d978aefefa83ec42d18dfa964ad180dbcde34795.1697116581.git.lstoakes@gmail.com open here

Signed-off-by Lorenzo Stoakes <[email protected]>

Reviewed-by Jan Kara <[email protected]>

Cc Alexander Viro <[email protected]>

Cc Christian Brauner <[email protected]>

Cc Hugh Dickins <[email protected]>

Cc Matthew Wilcox (Oracle) <[email protected]>

Cc Mike Kravetz <[email protected]>

Cc Muchun Song <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

---

 include/linux/fs.h |    4 ++--
 include/linux/mm.h |   11 +++++++++++
 kernel/fork.c      |    2 +-
 mm/filemap.c       |    2 +-
 mm/madvise.c       |    2 +-
 mm/mmap.c          |   12 ++++++------
 6 files changed, 22 insertions(+), 11 deletions(-)

include/linux/fs.h renamed from include/linux/fs.h~mm-drop-the-assumption-that-vm_shared-always-implies-writable +2 -2

@@ -454,7 +454,7 @@ extern const struct address_space_operat
  *   It is also used to block modification of page cache contents through
  *   memory mappings.
  * @gfp_mask: Memory allocation flags to use for allocating pages.
- * @i_mmap_writable: Number of VM_SHARED mappings.
+ * @i_mmap_writable: Number of VM_SHARED, VM_MAYWRITE mappings.
  * @nr_thps: Number of THPs in the pagecache (non-shmem only).
  * @i_mmap: Tree of private and shared mappings.
  * @i_mmap_rwsem: Protects @i_mmap and @i_mmap_writable.

@@ -557,7 +557,7 @@ static inline int mapping_mapped(struct
 
 /*
  * Might pages of this file have been modified in userspace?
- * Note that i_mmap_writable counts all VM_SHARED vmas: do_mmap
+ * Note that i_mmap_writable counts all VM_SHARED, VM_MAYWRITE vmas: do_mmap
  * marks vma as VM_SHARED if it is shared, and the file was opened for
  * writing i.e. vma may be mprotected writable even if now readonly.
  *

include/linux/mm.h renamed from include/linux/mm.h~mm-drop-the-assumption-that-vm_shared-always-implies-writable +11 -0

@@ -937,6 +937,17 @@ static inline bool vma_is_accessible(str
 	return vma->vm_flags & VM_ACCESS_FLAGS;
 }
 
+static inline bool is_shared_maywrite(vm_flags_t vm_flags)
+{
+	return (vm_flags & (VM_SHARED | VM_MAYWRITE)) ==
+		(VM_SHARED | VM_MAYWRITE);
+}
+
+static inline bool vma_is_shared_maywrite(struct vm_area_struct *vma)
+{
+	return is_shared_maywrite(vma->vm_flags);
+}
+
 static inline
 struct vm_area_struct *vma_find(struct vma_iterator *vmi, unsigned long max)
 {

kernel/fork.c renamed from kernel/fork.c~mm-drop-the-assumption-that-vm_shared-always-implies-writable +1 -1

@@ -733,7 +733,7 @@ static __latent_entropy int dup_mmap(str
 
 			get_file(file);
 			i_mmap_lock_write(mapping);
-			if (tmp->vm_flags & VM_SHARED)
+			if (vma_is_shared_maywrite(tmp))
 				mapping_allow_writable(mapping);
 			flush_dcache_mmap_lock(mapping);
 			/* insert tmp into the share list, just after mpnt */

mm/filemap.c renamed from mm/filemap.c~mm-drop-the-assumption-that-vm_shared-always-implies-writable +1 -1

@@ -3618,7 +3618,7 @@ int generic_file_mmap(struct file *file,
  */
 int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
+	if (vma_is_shared_maywrite(vma))
 		return -EINVAL;
 	return generic_file_mmap(file, vma);
 }

mm/madvise.c renamed from mm/madvise.c~mm-drop-the-assumption-that-vm_shared-always-implies-writable +1 -1

@@ -970,7 +970,7 @@ static long madvise_remove(struct vm_are
 			return -EINVAL;
 	}
 
-	if ((vma->vm_flags & (VM_SHARED|VM_WRITE)) != (VM_SHARED|VM_WRITE))
+	if (!vma_is_shared_maywrite(vma))
 		return -EACCES;
 
 	offset = (loff_t)(start - vma->vm_start)

mm/mmap.c renamed from mm/mmap.c~mm-drop-the-assumption-that-vm_shared-always-implies-writable +6 -6

@@ -107,7 +107,7 @@ void vma_set_page_prot(struct vm_area_st
 static void __remove_shared_vm_struct(struct vm_area_struct *vma,
 		struct file *file, struct address_space *mapping)
 {
-	if (vma->vm_flags & VM_SHARED)
+	if (vma_is_shared_maywrite(vma))
 		mapping_unmap_writable(mapping);
 
 	flush_dcache_mmap_lock(mapping);

@@ -384,7 +384,7 @@ static unsigned long count_vma_pages_ran
 static void __vma_link_file(struct vm_area_struct *vma,
 			    struct address_space *mapping)
 {
-	if (vma->vm_flags & VM_SHARED)
+	if (vma_is_shared_maywrite(vma))
 		mapping_allow_writable(mapping);
 
 	flush_dcache_mmap_lock(mapping);

@@ -2846,7 +2846,7 @@ cannot_expand:
 	vma->vm_pgoff = pgoff;
 
 	if (file) {
-		if (vm_flags & VM_SHARED) {
+		if (is_shared_maywrite(vm_flags)) {
 			error = mapping_map_writable(file->f_mapping);
 			if (error)
 				goto free_vma;

@@ -2920,7 +2920,7 @@ cannot_expand:
 	mm->map_count++;
 	if (vma->vm_file) {
 		i_mmap_lock_write(vma->vm_file->f_mapping);
-		if (vma->vm_flags & VM_SHARED)
+		if (vma_is_shared_maywrite(vma))
 			mapping_allow_writable(vma->vm_file->f_mapping);
 
 		flush_dcache_mmap_lock(vma->vm_file->f_mapping);

@@ -2937,7 +2937,7 @@ cannot_expand:
 
 	/* Once vma denies write, undo our temporary denial count */
 unmap_writable:
-	if (file && vm_flags & VM_SHARED)
+	if (file && is_shared_maywrite(vm_flags))
 		mapping_unmap_writable(file->f_mapping);
 	file = vma->vm_file;
 	ksm_add_vma(vma);

@@ -2985,7 +2985,7 @@ unmap_and_free_vma:
 		unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start,
 			     vma->vm_end, vma->vm_end, true);
 	}
-	if (file && (vm_flags & VM_SHARED))
+	if (file && is_shared_maywrite(vm_flags))
 		mapping_unmap_writable(file->f_mapping);
 free_vma:
 	vm_area_free(vma);

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