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Commit 6dbfc25d authored by Jann Horn's avatar Jann Horn Committed by Greg Kroah-Hartman
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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: 7a3ef208 ("mm: prevent endless growth of anon_vma hierarchy")
Cc: stable@kernel.org
Acked-by: default avatarMichal Hocko <mhocko@suse.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Signed-off-by: default avatarJann Horn <jannh@google.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent ddffe882

include/linux/rmap.h

+5 −2
Original line number Diff line number Diff line
@@ -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 */

mm/rmap.c

+17 −14
Original line number Diff line number Diff line
@@ -82,7 +82,8 @@ 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
@@ -190,6 +191,7 @@ 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;

	}
@@ -199,8 +201,7 @@ 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;

	}
@@ -279,19 +280,19 @@ 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 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 *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;
@@ -361,7 +363,7 @@ 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;
@@ -393,7 +395,7 @@ 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;

		}
@@ -401,7 +403,7 @@ 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--;

	unlock_anon_vma_root(root);



	/*
@@ -412,7 +414,8 @@ 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);