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;
Message-ID: <89d72aca4e30335a03322612cf164420be11d8eb.camel@redhat.com>
Subject: Re: [syzbot] INFO: rcu detected stall in sys_recvmmsg
From:   Paolo Abeni <pabeni@redhat.com>
To:     syzbot <syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com>,
        axboe@kernel.dk, christian.brauner@ubuntu.com, davem@davemloft.net,
        dkadashev@gmail.com, kuba@kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        mathew.j.martineau@linux.intel.com, matthieu.baerts@tessares.net,
        mptcp@lists.linux.dev, netdev@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, torvalds@linux-foundation.org,
        viro@zeniv.linux.org.uk
Date:   Wed, 22 Sep 2021 17:13:42 +0200
In-Reply-To: <0000000000003216d705cc8a62d6@google.com>
References: <0000000000003216d705cc8a62d6@google.com>
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) 
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Tue, 2021-09-21 at 17:13 -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    1f77990c4b79 Add linux-next specific files for 20210920
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1383891d300000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=ab1346371f2e6884
> dashboard link: https://syzkaller.appspot.com/bug?extid=3360da629681aa0d22fe
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1625f1ab300000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17eb1b3b300000
> 
> The issue was bisected to:
> 
> commit 020250f31c4c75ac7687a673e29c00786582a5f4
> Author: Dmitry Kadashev <dkadashev@gmail.com>
> Date:   Thu Jul 8 06:34:43 2021 +0000
> 
>     namei: make do_linkat() take struct filename
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15f5ef77300000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=17f5ef77300000
> console output: https://syzkaller.appspot.com/x/log.txt?x=13f5ef77300000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
> Fixes: 020250f31c4c ("namei: make do_linkat() take struct filename")

I'm unsure why the bisection points to this commit. This looks like an
MPTCP specific issue, due to bad handling of MSG_WAITALL recvmsg()
flag.

I'll test a fix with syzbot to confirm the above.

Cheers,

Paolo