linux-kernel - Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule

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Message-ID: <84b0471d-42c1-175f-ae1d-a18c310c7f77@codethink.co.uk>
Date:   Fri, 12 Mar 2021 13:49:53 +0000
From:   Ben Dooks <ben.dooks@...ethink.co.uk>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+e74b94fe601ab9552d69@...kaller.appspotmail.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Benjamin Segall <bsegall@...gle.com>, dietmar.eggemann@....com,
        Juri Lelli <juri.lelli@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [syzbot] BUG: unable to handle kernel access to user memory in
 schedule_tail

On 10/03/2021 17:16, Dmitry Vyukov wrote:
> On Wed, Mar 10, 2021 at 5:46 PM syzbot
> <syzbot+e74b94fe601ab9552d69@...kaller.appspotmail.com> wrote:
>>
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    0d7588ab riscv: process: Fix no prototype for arch_dup_tas..
>> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1212c6e6d00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
>> dashboard link: https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
>> userspace arch: riscv64
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+e74b94fe601ab9552d69@...kaller.appspotmail.com
> 
> +riscv maintainers
> 
> This is riscv64-specific.
> I've seen similar crashes in put_user in other places. It looks like
> put_user crashes in the user address is not mapped/protected (?).

I've been having a look, and this seems to be down to access of the
tsk->set_child_tid variable. I assume the fuzzing here is to pass a
bad address to clone?

 From looking at the code, the put_user() code should have set the
relevant SR_SUM bit (the value for this, which is 1<<18 is in the
s2 register in the crash report) and from looking at the compiler
output from my gcc-10, the code looks to be dong the relevant csrs
and then csrc around the put_user

So currently I do not understand how the above could have happened
over than something re-tried the code seqeunce and ended up retrying
the faulting instruction without the SR_SUM bit set.

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

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