linux-ext4 - Re: [syzbot] [ext4?] UBSAN: shift-out-of-bounds in ext2_fill

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Date:   Tue, 13 Jun 2023 23:05:26 +0200
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Aleksandr Nogikh <nogikh@...gle.com>, adilger.kernel@...ger.ca,
        jack@...e.com, linux-ext4@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+af5e10f73dbff48f70af@...kaller.appspotmail.com>
Subject: Re: [syzbot] [ext4?] UBSAN: shift-out-of-bounds in ext2_fill_super
 (2)

On Tue 13-06-23 14:01:03, Theodore Ts'o wrote:
> I wonder if we should have a separate syzkaller subsystem for ext2 (as
> distinct from ext4)?  The syz reproducer seems to know that it should
> be mounting using ext2, but also calls it an ext4 file system, which
> is a bit weird.  I'm guessing there is something specific about the
> syzkaller internals which might not make this be practical, but I
> thought I should ask.

Yeah, having ext2 driver as a separate subsystem makes sense to me since it
is completely different codebase.

> From the syz reproducer:
> 
> syz_mount_image$ext4(&(0x7f0000000100)='ext2\x00', ...)
> 
> More generally, there are a series of changes that were made to make
> ext4 to make it more robust against maliciously fuzzed superblocks,
> but we haven't necessarily made sure the same analogous changes have
> been made to ext2.  I'm not sure how critical this is in practice,
> since most distributions don't actually compile fs/ext2 and instead
> use CONFIG_EXT4_USE_FOR_EXT2 instead.  However, while we maintain ext2
> as a sample "simple" modern file system, I guess we should try to make
> sure we do carry those fixes over.
> 
> Jan, as the ext2 maintainer, do you have an opinion?

I agree, I try to fix these problems when syzbot finds them. For this one,
I've already sent a fix [1] (dropping remains of fragments support from ext2).

								Honza

[1] https://lore.kernel.org/all/20230613103012.22933-1-jack@suse.cz

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