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;
Subject: Re: [syzbot] BUG: unable to handle kernel access to user memory in
 schedule_tail
To:     Dmitry Vyukov <dvyukov@google.com>,
        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>
Cc:     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>
References: <000000000000b74f1b05bd316729@google.com>
 <CACT4Y+ZHMYijYAkeLMX=p9jx6pBivJz06h_1rGt-k9=AgfkWQg@mail.gmail.com>
From:   Ben Dooks <ben.dooks@codethink.co.uk>
Organization: Codethink Limited.
Message-ID: <84b0471d-42c1-175f-ae1d-a18c310c7f77@codethink.co.uk>
Date:   Fri, 12 Mar 2021 13:49:53 +0000
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.8.0
MIME-Version: 1.0
In-Reply-To: <CACT4Y+ZHMYijYAkeLMX=p9jx6pBivJz06h_1rGt-k9=AgfkWQg@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-GB
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 10/03/2021 17:16, Dmitry Vyukov wrote:
> On Wed, Mar 10, 2021 at 5:46 PM syzbot
> <syzbot+e74b94fe601ab9552d69@syzkaller.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@syzkaller.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

https://www.codethink.co.uk/privacy.html