linux-kernel - Re: proc_flush

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Date:   Tue, 19 Dec 2017 14:30:20 -0500
From:   Dave Jones <davej@...emonkey.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: proc_flush_task oops

On Tue, Dec 19, 2017 at 12:27:30PM -0600, Eric W. Biederman wrote:
 > Dave Jones <davej@...emonkey.org.uk> writes:
 > 
 > > On Mon, Dec 18, 2017 at 03:50:52PM -0800, Linus Torvalds wrote:
 > >
 > >  > But I don't see what would have changed in this area recently.
 > >  > 
 > >  > Do you end up saving the seeds that cause crashes? Is this
 > >  > reproducible? (Other than seeing it twoce, of course)
 > >
 > > Only clue so far, is every time I'm able to trigger it, the last thing
 > > the child process that triggers it did, was an execveat.
 > 
 > Is there any chance the excveat might be called from a child thread?

If trinity choose one of the exec syscalls, it forks off an extra child
to do it in, on the off-chance that it succeeds, and we never return.
https://github.com/kernelslacker/trinity/blob/master/syscall.c#L139

 > That switching pids between tasks of a process during exec can get a
 > little bit tricky.
 > 
 > > Telling it to just fuzz execveat doesn't instantly trigger it, so it
 > > must be a combination of some other syscall. I'll leave a script running
 > > overnight to see if I can binary search the other syscalls in
 > > combination with it.
 > 
 > Could we have a buggy syscall that is stomping something?

Not totally impossible I guess, though I would expect that would
manifest in additional random failures, whereas this seems remarkably
consistent.

	Dave