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Date:   Sun, 7 May 2023 20:18:16 +0200
From:   Jan Kara <jack@suse.cz>
To:     Theodore Ts'o <tytso@mit.edu>
Cc:     Ext4 Developers List <linux-ext4@vger.kernel.org>,
        syzbot+e2efa3efc15a1c9e95c3@syzkaller.appspotmail.com
Subject: Re: [PATCH 1/2] ext4: allow ext4_get_group_info() to fail
Message-ID: <20230507181816.tsnqhzgajftcbsz5@quack3>
References: <20230430154311.579720-1-tytso@mit.edu>
 <20230430154311.579720-2-tytso@mit.edu>
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On Sun 30-04-23 11:43:10, Theodore Ts'o wrote:
> Previously, ext4_get_group_info() would treat an invalid group number
> as BUG(), since in theory it should never happen.  However, if a
> malicious attaker (or fuzzer) modifies the superblock via the block
> device while it is the file system is mounted, it is possible for
> s_first_data_block to get set to a very large number.  In that case,
> when calculating the block group of some block number (such as the
> starting block of a preallocation region), could result in an
> underflow and very large block group number.  Then the BUG_ON check in
> ext4_get_group_info() would fire, resutling in a denial of service
> attack that can be triggered by root or someone with write access to
> the block device.
> 
> For a quality of implementation perspective, it's best that even if
> the system administrator does something that they shouldn't, that it
> will not trigger a BUG.  So instead of BUG'ing, ext4_get_group_info()
> will call ext4_error and return NULL.  We also add fallback code in
> all of the callers of ext4_get_group_info() that it might NULL.
> 
> Also, since ext4_get_group_info() was already borderline to be an
> inline function, un-inline it.  The results in a next reduction of the
> compiled text size of ext4 by roughly 2k.
> 
> Reported-by: syzbot+e2efa3efc15a1c9e95c3@syzkaller.appspotmail.com
> Link: https://syzkaller.appspot.com/bug?id=69b28112e098b070f639efb356393af3ffec4220
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>

The patch looks good except for one small problem already found by Julia:

> @@ -2578,7 +2595,7 @@ void ext4_mb_prefetch_fini(struct super_block *sb, ext4_group_t group,
>  		gdp = ext4_get_group_desc(sb, group, NULL);
>  		grp = ext4_get_group_info(sb, group);
>  
> -		if (EXT4_MB_GRP_NEED_INIT(grp) &&
> +		if (grp && grp && EXT4_MB_GRP_NEED_INIT(grp) &&
		    ^^^ one of these should be gdp.

With this fixed feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR