[PATCH 4.18 08/34] fscache: Fix out of bound read in long cookie keys

Greg Kroah-Hartman
2018-11-09T07:47:40Z

From: Eric Sandeen <sandeen@redhat.com>

commit fa520c47eaa15b9baa8ad66ac18da4a31679693b upstream.

fscache_set_key() can incur an out-of-bounds read, reported by KASAN:

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

and also reported by syzbot at https://lkml.org/lkml/2018/7/8/236

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.

Fix this by calculating how many u32 buffers we'll need by using
DIV_ROUND_UP, and then using kcalloc() to allocate a precleared allocation
buffer to hold the index_key, then using that same count as the hashing
index limit.

Fixes: ec0328e46d6e ("fscache: Maintain a catalogue of allocated cookies")
Reported-by: syzbot+a95b989b2dde8e806af8@syzkaller.appspotmail.com
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/fscache/cookie.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -70,7 +70,7 @@ void fscache_free_cookie(struct fscache_
}

/*
- * 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.
@@ -80,10 +80,13 @@ static int fscache_set_key(struct fscach
{
unsigned long long h;
u32 *buf;
+ int bufs;
int i;

+ 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 = kcalloc(bufs, sizeof(*buf), GFP_KERNEL);
if (!buf)
return -ENOMEM;
cookie->key = buf;
@@ -98,7 +101,8 @@ static int fscache_set_key(struct fscach
*/
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);