> On Nov 28, 2016, at 10:57 PM, Eryu Guan <guaneryu@xxxxxxxxx> wrote:
>
> 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@xxxxxxx>
> Signed-off-by: Eryu Guan <guaneryu@xxxxxxxxx>
Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
> ---
>
> In e2fsprogs, we avoided a similar buffer overrun:
> f66e6ce libext2fs: avoid buffer overflow if s_first_meta_bg is too big
>
> and e2fsck could detect & fix large s_first_meta_bg:
> 7a4352d e2fsck: fix file systems with an overly large s_first_meta_bg
>
> But I suspect that there's an off-by-one bug in e2fsck code, shouldn't the
> upper boundary of s_first_meta_bg be (fs->desc_blocks - 1)? Something like:
>
> e2fsck/super.c:643
> if (ext2fs_has_feature_meta_bg(fs->super) &&
> - (fs->super->s_first_meta_bg > fs->desc_blocks)) {
> - pctx.group = fs->desc_blocks;
> + (fs->super->s_first_meta_bg >= fs->desc_blocks)) {
> + pctx.group = fs->desc_blocks - 1;
> pctx.num = fs->super->s_first_meta_bg;
>
> fs/ext4/super.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 52b0530..8f46a07 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3814,6 +3814,15 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> (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_feature_meta_bg(sb)) {
> + 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);
> --
> 2.9.3
>
> --
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Cheers, Andreas
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