Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail

From: Dmitry Vyukov <dvyukov@google.com>
To: Ben Dooks <ben.dooks@codethink.co.uk>
Cc: Alex Ghiti <alex@ghiti.fr>,
	syzbot <syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Benjamin Segall <bsegall@google.com>,
	dietmar.eggemann@arm.com, Juri Lelli <juri.lelli@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, Mel Gorman <mgorman@suse.de>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail
Date: Thu, 18 Mar 2021 15:34:29 +0100	[thread overview]
Message-ID: <CACT4Y+aEAs2zrTY453mK2UgtaG=-eOyXyWU0PBSybmP+sc809A@mail.gmail.com> (raw)
In-Reply-To: <a0449f4a-4d54-6f38-1bc3-7c5170d24b86@codethink.co.uk>

On Mon, Mar 15, 2021 at 5:55 PM Ben Dooks <ben.dooks@codethink.co.uk> wrote:
> > On Fri, Mar 12, 2021 at 9:12 PM Ben Dooks <ben.dooks@codethink.co.uk> wrote:
>
> >>> Still no luck for the moment, can't reproduce it locally, my test is
> >>> maybe not that good (I created threads all day long in order to trigger
> >>> the put_user of schedule_tail).
> >>
> >> It may of course depend on memory and other stuff. I did try to see if
> >> it was possible to clone() with the child_tid address being a valid but
> >> not mapped page...
> >>
> >>> Given that the path you mention works most of the time, and that the
> >>> status register in the stack trace shows the SUM bit is not set whereas
> >>> it is set in put_user, I'm leaning toward some race condition (maybe an
> >>> interrupt that arrives at the "wrong" time) or a qemu issue as you
> >>> mentioned.
> >>
> >> I suppose this is possible. From what I read it should get to the
> >> point of being there with the SUM flag cleared, so either something
> >> went wrong in trying to fix the instruction up or there's some other
> >> error we're missing.
> >>
> >>> To eliminate qemu issues, do you have access to some HW ? Or to
> >>> different qemu versions ?
> >>
> >> I do have access to a Microchip Polarfire board. I just need the
> >> instructions on how to setup the test-code to make it work on the
> >> hardware.
> >
> > For full syzkaller support, it would need to know how to reboot these
> > boards and get access to the console.
> > syzkaller has a stop-gap VM backend which just uses ssh to a physical
> > machine and expects the kernel to reboot on its own after any crashes.
> >
> > But I actually managed to reproduce it in an even simpler setup.
> > Assuming you have Go 1.15 and riscv64 cross-compiler gcc installed
> >
> > $ go get -u -d github.com/google/syzkaller/...
> > $ cd $GOPATH/src/github.com/google/syzkaller
> > $ make stress executor TARGETARCH=riscv64
> > $ scp bin/linux_riscv64/syz-execprog bin/linux_riscv64/syz-executor
> > your_machine:/
> >
> > Then run ./syz-stress on the machine.
> > On the first run it crashed it with some other bug, on the second run
> > I got the crash in schedule_tail.
> > With qemu tcg I also added -slowdown=10 flag to syz-stress to scale
> > all timeouts, if native execution is faster, then you don't need it.
>
> I have built the tools and got it to start.
>
> It would be helpful for the dashboard to give the qemu version and
> how it was launched (memory, cpus etc)

Hi Ben,

syzbot will show info about qemu version/args in "VM info" column then
this commit is deployed (should happen by tomorrow);
https://github.com/google/syzkaller/commit/4a3131941837f1fab321bcdfcac13ac4fb480684