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From: Eric Sandeen <>
To: fsdevel <>
Cc: David Howells <>
Subject: [PATCH] fscache: Fix cookie key uninit mem / out of bound read
Date: Thu, 6 Sep 2018 10:31:11 -0500	[thread overview]
Message-ID: <> (raw)

fscache_set_key() seems to have 2 issues related to the memory
read for hashing in fscache_set_key.

The first reported was a KASAN error, 

 BUG: KASAN: slab-out-of-bounds in fscache_alloc_cookie+0x5b3/0x680 [fscache] 
 Read of size 4 at addr ffff88084ff056d4 by task mount.nfs/32615 

also reported by syzbot at

  BUG: KASAN: slab-out-of-bounds in fscache_set_key fs/fscache/cookie.c:120 [inline]
  BUG: KASAN: slab-out-of-bounds in fscache_alloc_cookie+0x7a9/0x880 fs/fscache/cookie.c:171
  Read of size 4 at addr ffff8801d3cc8bb4 by task syz-executor907/4466

This happens for any index_key_len which is not divisible by 4, and is
larger than the size of the inline key, because the code allocates exactly
index_key_len for the key buffer, but the hashing loop is stepping through
it 4 bytes (u32) at a time in the buf[] array.

When looking over this, it also appears that the inline key is
insufficiently initialized, zeroing only 3 of the 4 slots.  Hence
an index_key_len between 13 and 15 bytes will end up hashing uninitialized
memory because the memcpy only partially fills the last buf[] element.

Fix this by calculating how many u32 buffers we'll need by using
DIV_ROUND_UP, and memset them all to zero before copying in the index_key,
then using that same count as the hashing index limit.

Signed-off-by: Eric Sandeen <>

nb: compile-tested only, sorry.

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 83bfe04..bc74ae3 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -83,7 +83,7 @@ void fscache_cookie_init_once(void *_cookie)
- * Set the index key in a cookie.  The cookie struct has space for a 12-byte
+ * Set the index key in a cookie.  The cookie struct has space for a 16-byte
  * key plus length and hash, but if that's not big enough, it's instead a
  * pointer to a buffer containing 3 bytes of hash, 1 byte of length and then
  * the key data.
@@ -93,22 +93,22 @@ static int fscache_set_key(struct fscache_cookie *cookie,
 	unsigned long long h;
 	u32 *buf;
+	int bufs;
 	int i;
 	cookie->key_len = index_key_len;
+	bufs = DIV_ROUND_UP(index_key_len, sizeof(*buf));
 	if (index_key_len > sizeof(cookie->inline_key)) {
-		buf = kzalloc(index_key_len, GFP_KERNEL);
+		buf = kmalloc_array(bufs, sizeof(*buf), GFP_KERNEL);
 		if (!buf)
 			return -ENOMEM;
 		cookie->key = buf;
 	} else {
 		buf = (u32 *)cookie->inline_key;
-		buf[0] = 0;
-		buf[1] = 0;
-		buf[2] = 0;
+	memset(buf, 0, bufs * sizeof(*buf));
 	memcpy(buf, index_key, index_key_len);
 	/* Calculate a hash and combine this with the length in the first word
@@ -116,7 +116,8 @@ static int fscache_set_key(struct fscache_cookie *cookie,
 	h = (unsigned long)cookie->parent;
 	h += index_key_len + cookie->type;
-	for (i = 0; i < (index_key_len + sizeof(u32) - 1) / sizeof(u32); i++)
+	for (i = 0; i < bufs; i++)
 		h += buf[i];
 	cookie->key_hash = h ^ (h >> 32);

             reply	other threads:[~2018-09-06 20:07 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-06 15:31 Eric Sandeen [this message]
2018-09-25 19:25 ` [PATCH] fscache: Fix cookie key uninit mem / out of bound read Eric Sandeen

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