[PATCH 4.19 50/56] mm/rmap: Fix anon_vma->degree ambiguity leading to double-reuse

Greg Kroah-Hartman
2022-09-02T12:31:59Z

From: Jann Horn <jannh@google.com>

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>
---
include/linux/rmap.h | 7 +++++--
mm/rmap.c | 31 +++++++++++++++++--------------
2 files changed, 22 insertions(+), 16 deletions(-)

--- 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 */

--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -82,7 +82,8 @@ static inline struct anon_vma *anon_vma_
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
@@ -190,6 +191,7 @@ int __anon_vma_prepare(struct vm_area_st
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;
}

@@ -199,8 +201,7 @@ int __anon_vma_prepare(struct vm_area_st
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;
}
@@ -279,19 +280,19 @@ int anon_vma_clone(struct vm_area_struct
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 chose 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 && anon_vma != src->anon_vma &&
- anon_vma->degree < 2)
+ if (!dst->anon_vma &&
+ 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;

@@ -341,6 +342,7 @@ int anon_vma_fork(struct vm_area_struct
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;
@@ -361,7 +363,7 @@ int anon_vma_fork(struct vm_area_struct
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;
@@ -393,7 +395,7 @@ void unlink_anon_vmas(struct vm_area_str
* 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;
}

@@ -401,7 +403,7 @@ void unlink_anon_vmas(struct vm_area_str
anon_vma_chain_free(avc);
}
if (vma->anon_vma)
- vma->anon_vma->degree--;
+ vma->anon_vma->num_active_vmas--;
unlock_anon_vma_root(root);

/*
@@ -412,7 +414,8 @@ void unlink_anon_vmas(struct vm_area_str
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);