diff options
author | Jann Horn <jannh@google.com> | 2022-08-31 20:06:00 +0300 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2022-09-05 11:28:56 +0300 |
commit | 98f401d36396134c0c86e9e3bd00b6b6b028b521 (patch) | |
tree | 4ef9f5eded963c84e8970b4a3ddb21c313d9a33e /include/linux/rmap.h | |
parent | 6204bf78b2a903b96ba43afff6abc0b04d6e0462 (diff) | |
download | linux-98f401d36396134c0c86e9e3bd00b6b6b028b521.tar.xz |
mm/rmap: Fix anon_vma->degree ambiguity leading to double-reuse
commit 2555283eb40df89945557273121e9393ef9b542b upstream.
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include/linux/rmap.h')
-rw-r--r-- | include/linux/rmap.h | 7 |
1 files changed, 5 insertions, 2 deletions
diff --git a/include/linux/rmap.h b/include/linux/rmap.h index 8d04e7deedc6..297744ea4dd0 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -39,12 +39,15 @@ 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 */ |