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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 /mm/rmap.c
parentc5e4d5e99162ba8025d58a3af7ad103f155d2df7 (diff)
downloadlinux-2555283eb40df89945557273121e9393ef9b542b.tar.bz2
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 'mm/rmap.c')
-rw-r--r--mm/rmap.c29
1 files changed, 16 insertions, 13 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index edc06c52bc82..93d5a6f793d2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -93,7 +93,8 @@ static inline struct anon_vma *anon_vma_alloc(void)
anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
if (anon_vma) {
atomic_set(&anon_vma->refcount, 1);
- anon_vma->degree = 1; /* Reference for first vma */
+ anon_vma->num_children = 0;
+ anon_vma->num_active_vmas = 0;
anon_vma->parent = anon_vma;
/*
* Initialise the anon_vma root to point to itself. If called
@@ -201,6 +202,7 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
anon_vma = anon_vma_alloc();
if (unlikely(!anon_vma))
goto out_enomem_free_avc;
+ anon_vma->num_children++; /* self-parent link for new root */
allocated = anon_vma;
}
@@ -210,8 +212,7 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
if (likely(!vma->anon_vma)) {
vma->anon_vma = anon_vma;
anon_vma_chain_link(vma, avc, anon_vma);
- /* vma reference or self-parent link for new root */
- anon_vma->degree++;
+ anon_vma->num_active_vmas++;
allocated = NULL;
avc = NULL;
}
@@ -296,19 +297,19 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
anon_vma_chain_link(dst, avc, anon_vma);
/*
- * Reuse existing anon_vma if its degree lower than two,
- * that means it has no vma and only one anon_vma child.
+ * Reuse existing anon_vma if it has no vma and only one
+ * anon_vma child.
*
- * Do not choose parent anon_vma, otherwise first child
- * will always reuse it. Root anon_vma is never reused:
+ * Root anon_vma is never reused:
* it has self-parent reference and at least one child.
*/
if (!dst->anon_vma && src->anon_vma &&
- anon_vma != src->anon_vma && anon_vma->degree < 2)
+ anon_vma->num_children < 2 &&
+ anon_vma->num_active_vmas == 0)
dst->anon_vma = anon_vma;
}
if (dst->anon_vma)
- dst->anon_vma->degree++;
+ dst->anon_vma->num_active_vmas++;
unlock_anon_vma_root(root);
return 0;
@@ -358,6 +359,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
anon_vma = anon_vma_alloc();
if (!anon_vma)
goto out_error;
+ anon_vma->num_active_vmas++;
avc = anon_vma_chain_alloc(GFP_KERNEL);
if (!avc)
goto out_error_free_anon_vma;
@@ -378,7 +380,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
vma->anon_vma = anon_vma;
anon_vma_lock_write(anon_vma);
anon_vma_chain_link(vma, avc, anon_vma);
- anon_vma->parent->degree++;
+ anon_vma->parent->num_children++;
anon_vma_unlock_write(anon_vma);
return 0;
@@ -410,7 +412,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
* to free them outside the lock.
*/
if (RB_EMPTY_ROOT(&anon_vma->rb_root.rb_root)) {
- anon_vma->parent->degree--;
+ anon_vma->parent->num_children--;
continue;
}
@@ -418,7 +420,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
anon_vma_chain_free(avc);
}
if (vma->anon_vma) {
- vma->anon_vma->degree--;
+ vma->anon_vma->num_active_vmas--;
/*
* vma would still be needed after unlink, and anon_vma will be prepared
@@ -436,7 +438,8 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
struct anon_vma *anon_vma = avc->anon_vma;
- VM_WARN_ON(anon_vma->degree);
+ VM_WARN_ON(anon_vma->num_children);
+ VM_WARN_ON(anon_vma->num_active_vmas);
put_anon_vma(anon_vma);
list_del(&avc->same_vma);