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 [manually fixed up different indentation in stable] Signed-off-by: Jann Horn jannh@google.com 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 @@ -37,12 +37,15 @@ 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 @@ -78,7 +78,8 @@ 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 @@ -187,6 +188,7 @@ int anon_vma_prepare(struct vm_area_stru 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; }
@@ -196,8 +198,7 @@ int anon_vma_prepare(struct vm_area_stru 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; } @@ -276,19 +277,19 @@ 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;
@@ -338,6 +339,7 @@ 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; @@ -358,7 +360,7 @@ 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; @@ -390,7 +392,7 @@ void unlink_anon_vmas(struct vm_area_str * to free them outside the lock. */ if (RB_EMPTY_ROOT(&anon_vma->rb_root)) { - anon_vma->parent->degree--; + anon_vma->parent->num_children--; continue; }
@@ -398,7 +400,7 @@ 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);
/* @@ -409,7 +411,8 @@ 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);