linux-kernel - Re: INFO: task hung in xlog_grant_head

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Date:   Wed, 23 May 2018 13:01:59 -0500
From:   Eric Sandeen <sandeen@...deen.net>
To:     Eric Biggers <ebiggers3@...il.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Dave Chinner <david@...morbit.com>,
        Brian Foster <bfoster@...hat.com>,
        syzbot <syzbot+568245b88fbaedcb1959@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: INFO: task hung in xlog_grant_head_check

On 5/23/18 11:20 AM, Eric Biggers wrote:
> Hi Darrick,

...

> Now, if you *really* don't want syzbot to report XFS bugs as you believe XFS
> contains known unfixable bugs or for other reasons, you can formally ask Dmitry
> to remove CONFIG_XFS_FS from the syzbot config.  But of course that doesn't make
> the bugs go away, it just makes the bug reports go away; you'll have to fix them
> eventually anyway, one way or another.

I'd revise that to "have to fix /some/ of them anyway."

What I'm personally hung up on are the bugs where the "exploit" involves merely
mounting a crafted filesystem that in reality would never (until the heat death
of the universe) corrupt itself into that state on its own; it's the "malicious
image" case, which is quite different than exposing fundamental bugs like the
SB_BORN race or or the user-exploitable ext4 flaw you mentioned in your reply.
Those are more insidious and/or things which can be hit by real users in real life.

I don't know if I can win the "malicious images aren't a critical security
threat" battle, but I do think they are at least a different class of flaws,
because as Dave said, mount is supposed to be a privileged operation.
In a perfect world we'd fix them anyway, but I don't know that our resource
pool can keep up with your google-scale bot and still make progress in other
critical areas.

Anyway, the upshot is that we're probably just not going to care much about V4
filesystem oops-or-hang-on-mount bugs.  Those problems are solved (largely) with
V5 filesystem format.  Maybe I /will/ propose a system-wide tunable to disallow
V4 for those who are worried about such things.

To Darrick's points about more collaboration, I still wish that our requests
for more traditional fs fuzzer reporting (i.e. a filesystem image) weren't met
with such resistance.Tailoring your bug reports to the needs of the developer
community you're interacting with seems like a pretty reasonable thing to do.

As an aside, I wonder how much coverage of the V5 format code syzkaller /has/
achieved; that would be another useful datapoint google could provide - if
syzkaller is in fact traversing the V5 codepaths and isn't turning anything
up, that'd be pretty useful to know.

Thanks,
-Eric