path: root/include/linux/rmap.h
diff options
authorJann Horn <jannh@google.com>2022-08-31 19:06:00 +0200
committerLinus Torvalds <torvalds@linux-foundation.org>2022-08-31 15:45:10 -0700
commit2555283eb40df89945557273121e9393ef9b542b (patch)
treeef4abf2ce0cf865cd5a07e5875f0e17853a51a00 /include/linux/rmap.h
parentMerge tag 'fscache-fixes-20220831' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs (diff)
mm/rmap: Fix anon_vma->degree ambiguity leading to double-reuse
anon_vma->degree tracks the combined number of child anon_vmas and VMAs that use the anon_vma as their ->anon_vma. anon_vma_clone() then assumes that for any anon_vma attached to src->anon_vma_chain other than src->anon_vma, it is impossible for it to be a leaf node of the VMA tree, meaning that for such VMAs ->degree is elevated by 1 because of a child anon_vma, meaning that if ->degree equals 1 there are no VMAs that use the anon_vma as their ->anon_vma. This assumption is wrong because the ->degree optimization leads to leaf nodes being abandoned on anon_vma_clone() - an existing anon_vma is reused and no new parent-child relationship is created. So it is possible to reuse an anon_vma for one VMA while it is still tied to another VMA. This is an issue because is_mergeable_anon_vma() and its callers assume that if two VMAs have the same ->anon_vma, the list of anon_vmas attached to the VMAs is guaranteed to be the same. When this assumption is violated, vma_merge() can merge pages into a VMA that is not attached to the corresponding anon_vma, leading to dangling page->mapping pointers that will be dereferenced during rmap walks. Fix it by separately tracking the number of child anon_vmas and the number of VMAs using the anon_vma as their ->anon_vma. Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy") Cc: stable@kernel.org Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Jann Horn <jannh@google.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to '')
1 files changed, 5 insertions, 2 deletions
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index bf80adca980b..b89b4b86951f 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -41,12 +41,15 @@ struct anon_vma {
atomic_t refcount;
- * Count of child anon_vmas and VMAs which points to this anon_vma.
+ * Count of child anon_vmas. Equals to the count of all anon_vmas that
+ * have ->parent pointing to this one, including itself.
* This counter is used for making decision about reusing anon_vma
* instead of forking new one. See comments in function anon_vma_clone.
- unsigned degree;
+ unsigned long num_children;
+ /* Count of VMAs whose ->anon_vma pointer points to this object. */
+ unsigned long num_active_vmas;
struct anon_vma *parent; /* Parent of this anon_vma */