[PATCHv2] fs/nilfs2: Fix potential underflow in call to crc32_le

[Torsten Hilbrich <torsten.hilbrich-opNxpl+3fjRBDgjK7y7TUQ@public.gmane.org>]

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.

[201699.185465] BUG: unable to handle kernel paging request at ffff88021e600000
[201699.186111] IP: [<ffffffff814083c6>] crc32_le+0x36/0x100
...
[201699.206202] Call Trace:
[201699.206982]  [<ffffffffc0907492>] nilfs_valid_sb.part.5+0x52/0x60 [nilfs2]
[201699.207773]  [<ffffffffc09075e2>] nilfs_load_super_block+0x142/0x300 [nilfs2]
[201699.208564]  [<ffffffff812479fd>] ? set_blocksize+0x9d/0xd0
[201699.209355]  [<ffffffffc0908020>] init_nilfs+0x60/0x390 [nilfs2]
[201699.210160]  [<ffffffffc08fc962>] nilfs_mount+0x302/0x520 [nilfs2]
[201699.210930]  [<ffffffff811b16a5>] ? pcpu_alloc+0x385/0x670
[201699.211685]  [<ffffffff81210c58>] mount_fs+0x38/0x160
[201699.212413]  [<ffffffff811b19c5>] ? __alloc_percpu+0x15/0x20
[201699.213151]  [<ffffffff8122cbe7>] vfs_kern_mount+0x67/0x110
[201699.213898]  [<ffffffff8122f3b9>] do_mount+0x269/0xe00
[201699.214671]  [<ffffffff8122d5a4>] ? mntput+0x24/0x40
[201699.215432]  [<ffffffff811ef064>] ? __kmalloc_track_caller+0x1b4/0x250
[201699.216207]  [<ffffffff8120eaf0>] ? __fput+0x190/0x220
[201699.216987]  [<ffffffff811ac0e2>] ? memdup_user+0x42/0x70
[201699.217777]  [<ffffffff8123027f>] SyS_mount+0x9f/0x100
[201699.218595]  [<ffffffff81825bf2>] entry_SYSCALL_64_fastpath+0x16/0x71

Signed-off-by: Torsten Hilbrich <torsten.hilbrich-opNxpl+3fjRBDgjK7y7TUQ@public.gmane.org>
Tested-by: Torsten Hilbrich <torsten.hilbrich-opNxpl+3fjRBDgjK7y7TUQ@public.gmane.org>
---
 fs/nilfs2/the_nilfs.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 69bd801..2de63db 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -438,18 +438,19 @@ static int nilfs_valid_sb(struct nilfs_super_block *sbp)
 	static unsigned char sum[4];
 	const int sumoff = offsetof(struct nilfs_super_block, s_sum);
 	size_t bytes;
+	const size_t crc_start = sumoff + 4;
 	u32 crc;
 
 	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 < crc_start || bytes > BLOCK_SIZE)
 		return 0;
 	crc = crc32_le(le32_to_cpu(sbp->s_crc_seed), (unsigned char *)sbp,
 		       sumoff);
 	crc = crc32_le(crc, sum, 4);
-	crc = crc32_le(crc, (unsigned char *)sbp + sumoff + 4,
-		       bytes - sumoff - 4);
+	crc = crc32_le(crc, (unsigned char *)sbp + crc_start,
+		       bytes - crc_start);
 	return crc == le32_to_cpu(sbp->s_sum);
 }
 
-- 
2.1.4


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