linux-kernel - Re: [syzbot] [crypto?] KMSAN: uninit-value in __crc32c_le

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Date: Fri, 15 Dec 2023 15:41:49 +0100
From: Alexander Potapenko <glider@...gle.com>
To: Dave Chinner <david@...morbit.com>
Cc: Dave Chinner <dchinner@...hat.com>, 
	syzbot+a6d6b8fffa294705dbd8@...kaller.appspotmail.com, hch@....de, 
	davem@...emloft.net, herbert@...dor.apana.org.au, 
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com, linux-xfs@...r.kernel.org
Subject: Re: [syzbot] [crypto?] KMSAN: uninit-value in __crc32c_le_base (3)

On Thu, Dec 14, 2023 at 10:39 PM 'Dave Chinner' via syzkaller-bugs
<syzkaller-bugs@...glegroups.com> wrote:
>
> On Thu, Dec 14, 2023 at 03:55:00PM +0100, Alexander Potapenko wrote:
> > On Wed, Dec 13, 2023 at 10:58 PM 'Dave Chinner' via syzkaller-bugs
> > <syzkaller-bugs@...glegroups.com> wrote:
> > >
> > > On Thu, Dec 14, 2023 at 08:16:07AM +1100, Dave Chinner wrote:
> > > > [cc linux-xfs@...r.kernel.org because that's where all questions
> > > > about XFS stuff should be directed, not to random individual
> > > > developers. ]
> > > >
> > > > On Wed, Dec 13, 2023 at 11:49:50AM +0100, Alexander Potapenko wrote:
> > > > > Hi Christoph, Dave,
> > > > >
> > > > > The repro provided by Xingwei indeed works.
> > >
> > > Can you please test the patch below?
> >
> > It fixed the problem for me, feel free to add:
> >
> > Tested-by: Alexander Potapenko <glider@...gle.com>
>
> Thanks.
>
> > As for the time needed to detect the bug, note that kmemcheck was
> > never used together with syzkaller, so it couldn't have the chance to
> > find it.
> >
> > KMSAN found this bug in April
> > (https://syzkaller.appspot.com/bug?extid=a6d6b8fffa294705dbd8),
>
> KMSAN has been used for quite a long time with syzbot, however,
> and it's supposed to find these problems, too. Yet it's only been
> finding this for 6 months?
>
> > only
> > half a year after we started mounting XFS images on syzbot.
>
> Really? Where did you get that from?  syzbot has been exercising XFS
> filesystems since 2017 - the bug reports to the XFS list go back at
> least that far.

You are right, syzbot used to mount XFS way before 2022.
On the other hand, last fall there were some major changes to the way
syz_mount_image() works, so I am attributing the newly detected bugs
to those changes.
Unfortunately we don't have much insight into reasons behind syzkaller
being able to trigger one bug or another: once a bug is found for the
first time, the likelihood to trigger it again increases, but finding
it initially might be tricky.

I don't understand much how trivial is the repro at
https://gist.github.com/xrivendell7/c7bb6ddde87a892818ed1ce206a429c4,
but overall we are not drilling deep enough into XFS.
https://storage.googleapis.com/syzbot-assets/8547e3dd1cca/ci-upstream-kmsan-gce-c7402612.html
(ouch, 230Mb!) shows very limited coverage.