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fscache: Fix out of bound read in long cookie keys

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>

authored by

Eric Sandeen and committed by
Greg Kroah-Hartman fa520c47 1ff22883

+7 -3
+7 -3
fs/fscache/cookie.c
··· 70 70 } 71 71 72 72 /* 73 - * Set the index key in a cookie. The cookie struct has space for a 12-byte 73 + * Set the index key in a cookie. The cookie struct has space for a 16-byte 74 74 * key plus length and hash, but if that's not big enough, it's instead a 75 75 * pointer to a buffer containing 3 bytes of hash, 1 byte of length and then 76 76 * the key data. ··· 80 80 { 81 81 unsigned long long h; 82 82 u32 *buf; 83 + int bufs; 83 84 int i; 84 85 86 + bufs = DIV_ROUND_UP(index_key_len, sizeof(*buf)); 87 + 85 88 if (index_key_len > sizeof(cookie->inline_key)) { 86 - buf = kzalloc(index_key_len, GFP_KERNEL); 89 + buf = kcalloc(bufs, sizeof(*buf), GFP_KERNEL); 87 90 if (!buf) 88 91 return -ENOMEM; 89 92 cookie->key = buf; ··· 101 98 */ 102 99 h = (unsigned long)cookie->parent; 103 100 h += index_key_len + cookie->type; 104 - for (i = 0; i < (index_key_len + sizeof(u32) - 1) / sizeof(u32); i++) 101 + 102 + for (i = 0; i < bufs; i++) 105 103 h += buf[i]; 106 104 107 105 cookie->key_hash = h ^ (h >> 32);