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

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Message-ID: <ZIudnDI5JZU+4w42@dread.disaster.area>
Date:   Fri, 16 Jun 2023 09:24:12 +1000
From:   Dave Chinner <david@...morbit.com>
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, Jun 13, 2023 at 02:01:03PM -0400, 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.
> 
> 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.

Hmmmm.

Modern filesystems are crash resilient, based on extents and are
using/moving to folios+iomap - calling a non-journalled, indirect
block indexed, bufferhead based code base (that nobody is really
using in production) "modern" seems like a real stretch.

I have my doubts that maintaining fs/ext2 is providing much benefit
to anyone.  The code base is in the git history if anyone wants to
study it, so it's not like we have to keep it active in the tree for
it to remain a code base that people can learn from.

Therefore, given the current push to sideline/remove bufferheads
from the kernel, should we simply deprecate fs/ext2 and then remove
it in a year or two like we're doing with reiser to reduce our
future maintenance and/or conversion burden?

Or just remove it right now and simply make CONFIG_FS_EXT2 select
CONFIG_EXT4_USE_FOR_EXT2?

/Devil's Advocate

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com