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Subject[PATCH 3.2 52/61] ext4: validate s_first_meta_bg at mount time
3.2.96-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eryu Guan <guaneryu@gmail.com>

commit 3a4b77cd47bb837b8557595ec7425f281f2ca1fe upstream.

Ralf Spenneberg reported that he hit a kernel crash when mounting a
modified ext4 image. And it turns out that kernel crashed when
calculating fs overhead (ext4_calculate_overhead()), this is because
the image has very large s_first_meta_bg (debug code shows it's
842150400), and ext4 overruns the memory in count_overhead() when
setting bitmap buffer, which is PAGE_SIZE.

ext4_calculate_overhead():
buf = get_zeroed_page(GFP_NOFS); <=== PAGE_SIZE buffer
blks = count_overhead(sb, i, buf);

count_overhead():
for (j = ext4_bg_num_gdb(sb, grp); j > 0; j--) { <=== j = 842150400
ext4_set_bit(EXT4_B2C(sbi, s++), buf); <=== buffer overrun
count++;
}

This can be reproduced easily for me by this script:

#!/bin/bash
rm -f fs.img
mkdir -p /mnt/ext4
fallocate -l 16M fs.img
mke2fs -t ext4 -O bigalloc,meta_bg,^resize_inode -F fs.img
debugfs -w -R "ssv first_meta_bg 842150400" fs.img
mount -o loop fs.img /mnt/ext4

Fix it by validating s_first_meta_bg first at mount time, and
refusing to mount if its value exceeds the largest possible meta_bg
number.

Reported-by: Ralf Spenneberg <ralf@os-t.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
[bwh: Backported to 3.2: open-code ext4_has_feature_meta_bg()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/ext4/super.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3692,6 +3692,15 @@ static int ext4_fill_super(struct super_
(EXT4_MAX_BLOCK_FILE_PHYS / EXT4_BLOCKS_PER_GROUP(sb)));
db_count = (sbi->s_groups_count + EXT4_DESC_PER_BLOCK(sb) - 1) /
EXT4_DESC_PER_BLOCK(sb);
+ if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_META_BG)) {
+ if (le32_to_cpu(es->s_first_meta_bg) >= db_count) {
+ ext4_msg(sb, KERN_WARNING,
+ "first meta block group too large: %u "
+ "(group descriptor block count %u)",
+ le32_to_cpu(es->s_first_meta_bg), db_count);
+ goto failed_mount;
+ }
+ }
sbi->s_group_desc = ext4_kvmalloc(db_count *
sizeof(struct buffer_head *),
GFP_KERNEL);
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