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

[Ben Dooks]

On 13/03/2021 07:20, Dmitry Vyukov wrote:
> On Fri, Mar 12, 2021 at 9:12 PM Ben Dooks <ben.dooks at 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)

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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