[PATCH 3.16 229/305] fs/nilfs2: fix potential underflow in call to crc32_le

Ben Hutchings
2016-08-14T11:58:58Z

From: Torsten Hilbrich <torsten.hilbrich@secunet.com>

commit 63d2f95d63396059200c391ca87161897b99e74a upstream.

The value `bytes' comes from the filesystem which is about to be
mounted. We cannot trust that the value is always in the range we
expect it to be.

Check its value before using it to calculate the length for the crc32_le
call. It value must be larger (or equal) sumoff + 4.

This fixes a kernel bug when accidentially mounting an image file which
had the nilfs2 magic value 0x3434 at the right offset 0x406 by chance.
The bytes 0x01 0x00 were stored at 0x408 and were interpreted as a
s_bytes value of 1. This caused an underflow when substracting sumoff +
4 (20) in the call to crc32_le.

BUG: unable to handle kernel paging request at ffff88021e600000
IP: crc32_le+0x36/0x100
...
Call Trace:
nilfs_valid_sb.part.5+0x52/0x60 [nilfs2]
nilfs_load_super_block+0x142/0x300 [nilfs2]
init_nilfs+0x60/0x390 [nilfs2]
nilfs_mount+0x302/0x520 [nilfs2]
mount_fs+0x38/0x160
vfs_kern_mount+0x67/0x110
do_mount+0x269/0xe00
SyS_mount+0x9f/0x100
entry_SYSCALL_64_fastpath+0x16/0x71

Link: http://lkml.kernel.org/r/1466778587-5184-2-git-send-email-konishi.ryusuke@lab.ntt.co.jp
Signed-off-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Tested-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/nilfs2/the_nilfs.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -441,7 +441,7 @@ static int nilfs_valid_sb(struct nilfs_s
if (!sbp || le16_to_cpu(sbp->s_magic) != NILFS_SUPER_MAGIC)
return 0;
bytes = le16_to_cpu(sbp->s_bytes);
- if (bytes > BLOCK_SIZE)
+ if (bytes < sumoff + 4 || bytes > BLOCK_SIZE)
return 0;
crc = crc32_le(le32_to_cpu(sbp->s_crc_seed), (unsigned char *)sbp,
sumoff);