Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <000000000000b74f1b05bd316729@google.com> <CACT4Y+ZHMYijYAkeLMX=p9jx6pBivJz06h_1rGt-k9=AgfkWQg@mail.gmail.com>
 <84b0471d-42c1-175f-ae1d-a18c310c7f77@codethink.co.uk> <CACT4Y+ZsSRdQ5LzYMsgjrBAukgP-Vv8WSQsSoxguYjWvB1QnrA@mail.gmail.com>
 <795597a1-ec87-e09e-d073-3daf10422abb@ghiti.fr> <f9318e58-5d3f-f548-3dbf-a5cb87406a94@codethink.co.uk>
 <CACT4Y+bYDU-4MYotLfqgkGpz487aC8xQ0dqaNLwf76d1yyF-Vw@mail.gmail.com> <a0449f4a-4d54-6f38-1bc3-7c5170d24b86@codethink.co.uk>
In-Reply-To: <a0449f4a-4d54-6f38-1bc3-7c5170d24b86@codethink.co.uk>
From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 18 Mar 2021 15:34:29 +0100
Message-ID: <CACT4Y+aEAs2zrTY453mK2UgtaG=-eOyXyWU0PBSybmP+sc809A@mail.gmail.com>
Subject: Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail
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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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