linux-kernel - [PATCH 2/5] mm: convert anon_vma.refcount from atomic_t to refcount

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Date:   Mon, 20 Feb 2017 12:49:11 +0200
From:   Elena Reshetova <elena.reshetova@...el.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, peterz@...radead.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk,
        catalin.marinas@....com, mingo@...hat.com,
        akpm@...ux-foundation.org, arnd@...db.de, luto@...nel.org,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: [PATCH 2/5] mm: convert anon_vma.refcount from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
Signed-off-by: Hans Liljestrand <ishkamiel@...il.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
Signed-off-by: David Windsor <dwindsor@...il.com>
---
 include/linux/rmap.h |  7 ++++---
 mm/rmap.c            | 14 +++++++-------
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 8c89e90..a8f4a97 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -10,6 +10,7 @@
 #include <linux/rwsem.h>
 #include <linux/memcontrol.h>
 #include <linux/highmem.h>
+#include <linux/refcount.h>
 
 /*
  * The anon_vma heads a list of private "related" vmas, to scan if
@@ -35,7 +36,7 @@ struct anon_vma {
 	 * the reference is responsible for clearing up the
 	 * anon_vma if they are the last user on release
 	 */
-	atomic_t refcount;
+	refcount_t refcount;
 
 	/*
 	 * Count of child anon_vmas and VMAs which points to this anon_vma.
@@ -102,14 +103,14 @@ enum ttu_flags {
 #ifdef CONFIG_MMU
 static inline void get_anon_vma(struct anon_vma *anon_vma)
 {
-	atomic_inc(&anon_vma->refcount);
+	refcount_inc(&anon_vma->refcount);
 }
 
 void __put_anon_vma(struct anon_vma *anon_vma);
 
 static inline void put_anon_vma(struct anon_vma *anon_vma)
 {
-	if (atomic_dec_and_test(&anon_vma->refcount))
+	if (refcount_dec_and_test(&anon_vma->refcount))
 		__put_anon_vma(anon_vma);
 }
 
diff --git a/mm/rmap.c b/mm/rmap.c
index 8774791..3321c86 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -77,7 +77,7 @@ 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);
+		refcount_set(&anon_vma->refcount, 1);
 		anon_vma->degree = 1;	/* Reference for first vma */
 		anon_vma->parent = anon_vma;
 		/*
@@ -92,7 +92,7 @@ static inline struct anon_vma *anon_vma_alloc(void)
 
 static inline void anon_vma_free(struct anon_vma *anon_vma)
 {
-	VM_BUG_ON(atomic_read(&anon_vma->refcount));
+	VM_BUG_ON(refcount_read(&anon_vma->refcount));
 
 	/*
 	 * Synchronize against page_lock_anon_vma_read() such that
@@ -421,7 +421,7 @@ static void anon_vma_ctor(void *data)
 	struct anon_vma *anon_vma = data;
 
 	init_rwsem(&anon_vma->rwsem);
-	atomic_set(&anon_vma->refcount, 0);
+	refcount_set(&anon_vma->refcount, 0);
 	anon_vma->rb_root = RB_ROOT;
 }
 
@@ -470,7 +470,7 @@ struct anon_vma *page_get_anon_vma(struct page *page)
 		goto out;
 
 	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
-	if (!atomic_inc_not_zero(&anon_vma->refcount)) {
+	if (!refcount_inc_not_zero(&anon_vma->refcount)) {
 		anon_vma = NULL;
 		goto out;
 	}
@@ -529,7 +529,7 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
 	}
 
 	/* trylock failed, we got to sleep */
-	if (!atomic_inc_not_zero(&anon_vma->refcount)) {
+	if (!refcount_inc_not_zero(&anon_vma->refcount)) {
 		anon_vma = NULL;
 		goto out;
 	}
@@ -544,7 +544,7 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
 	rcu_read_unlock();
 	anon_vma_lock_read(anon_vma);
 
-	if (atomic_dec_and_test(&anon_vma->refcount)) {
+	if (refcount_dec_and_test(&anon_vma->refcount)) {
 		/*
 		 * Oops, we held the last refcount, release the lock
 		 * and bail -- can't simply use put_anon_vma() because
@@ -1577,7 +1577,7 @@ void __put_anon_vma(struct anon_vma *anon_vma)
 	struct anon_vma *root = anon_vma->root;
 
 	anon_vma_free(anon_vma);
-	if (root != anon_vma && atomic_dec_and_test(&root->refcount))
+	if (root != anon_vma && refcount_dec_and_test(&root->refcount))
 		anon_vma_free(root);
 }
 
-- 
2.7.4