Re: [PATCH] ext4: fix kernel BUG in ext4_free_blocks

[Lukas Czerner <lczerner@redhat.com>]

On Wed, Jul 13, 2022 at 11:59:04AM -0700, Tadeusz Struk wrote:
> Syzbot reported a BUG in ext4_free_blocks.
> The issue is triggered from ext4_mb_clear_bb(). What happens is the
> block number passed to ext4_get_group_no_and_offset() is 0 and the
> es->s_first_data_block is 1. This makes block group number returned
> from ext4_get_group_no_and_offset equal to -1. This is then passed to
> ext4_get_group_info() and hits a BUG:
> BUG_ON(group >= EXT4_SB(sb)->s_groups_count),
> what can be seen in the trace below.
> This patch adds an assertion to ext4_get_group_no_and_offset() that
> checks if block number is not smaller than es->s_first_data_block.
> 
> kernel BUG at fs/ext4/ext4.h:3319!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 0 PID: 337 Comm: repro Not tainted 5.19.0-rc6-00105-g4e8e898e4107-dirty #14
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.fc36 04/01/2014
> RIP: 0010:ext4_mb_clear_bb+0x1bd6/0x1be0
> Call Trace:
>  <TASK>
>  ext4_free_blocks+0x9b3/0xc90
>  ext4_clear_blocks+0x344/0x3b0
>  ext4_ind_truncate+0x967/0x1050
>  ext4_truncate+0xb1b/0x1210
>  ext4_evict_inode+0xf06/0x16f0
>  evict+0x2a3/0x630
>  iput+0x618/0x850
>  ext4_enable_quotas+0x578/0x920
>  ext4_orphan_cleanup+0x539/0x1200
>  ext4_fill_super+0x94d8/0x9bc0
>  get_tree_bdev+0x40c/0x630
>  ext4_get_tree+0x1c/0x20
>  vfs_get_tree+0x88/0x290
>  do_new_mount+0x289/0xac0
>  path_mount+0x607/0xfd0
>  __se_sys_mount+0x2c4/0x3b0
>  __x64_sys_mount+0xbf/0xd0
>  do_syscall_64+0x3d/0x90
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>  </TASK>
> 
> Link: https://syzkaller.appspot.com/bug?id=5266d464285a03cee9dbfda7d2452a72c3c2ae7c
> Reported-by: syzbot+15cd994e273307bf5cfa@syzkaller.appspotmail.com
> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
> ---
>  fs/ext4/balloc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 78ee3ef795ae..1175750ad05f 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -56,6 +56,9 @@ void ext4_get_group_no_and_offset(struct super_block *sb, ext4_fsblk_t blocknr,
>  	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
>  	ext4_grpblk_t offset;
>  
> +	if (blocknr < le32_to_cpu(es->s_first_data_block))
> +		blocknr = le32_to_cpu(es->s_first_data_block);
> +

This does not seem right. we should never work with block number smaller
than s_first_data_block. The first 1024 bytes of the file system are
unused and in case we have 1k block size, the entire first block is
unused.

I guess the image we work here with is corrupted, from the log it seems
that it was noticed correctly so the question is why did we still ended
up calling ext4_free_blocks() ? Seems like this should have been stopped
earlier by ext4_clear_blocks() ?

I did notice that in ext4_mb_clear_bb() we call
ext4_get_group_no_and_offset() before ext4_inode_block_valid() but
again we should have caught this problem earlier.

Can you link me the file system image that generated this problem?

Thanks!
-Lukas


>  	blocknr = blocknr - le32_to_cpu(es->s_first_data_block);
>  	offset = do_div(blocknr, EXT4_BLOCKS_PER_GROUP(sb)) >>
>  		EXT4_SB(sb)->s_cluster_bits;
> -- 
> 2.36.1
>