linux-kernel - Re: possible deadlock in free_ioctx

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Date:   Mon, 10 Sep 2018 11:43:31 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     syzbot <syzbot+d86c4426a01f60feddc7@...kaller.appspotmail.com>,
        bcrl <bcrl@...ck.org>, linux-aio <linux-aio@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: possible deadlock in free_ioctx_users

On Mon, Sep 10, 2018 at 11:28 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Sun, Sep 9, 2018 at 8:41 PM, syzbot
> <syzbot+d86c4426a01f60feddc7@...kaller.appspotmail.com> wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    f8f65382c98a Merge tag 'for-linus' of git://git.kernel.org..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=113260ae400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=8f59875069d721b6
>> dashboard link: https://syzkaller.appspot.com/bug?extid=d86c4426a01f60feddc7
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=120baa9e400000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13979cbe400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+d86c4426a01f60feddc7@...kaller.appspotmail.com
>>
>> random: sshd: uninitialized urandom read (32 bytes read)
>> random: sshd: uninitialized urandom read (32 bytes read)
>> random: sshd: uninitialized urandom read (32 bytes read)
>>
>> ========================================================
>> WARNING: possible irq lock inversion dependency detected
>> 4.19.0-rc2+ #229 Not tainted
>> --------------------------------------------------------
>> swapper/0/0 just changed the state of lock:
>> 00000000c02bddef (&(&ctx->ctx_lock)->rlock){..-.}, at: spin_lock_irq
>> include/linux/spinlock.h:354 [inline]
>> 00000000c02bddef (&(&ctx->ctx_lock)->rlock){..-.}, at:
>> free_ioctx_users+0xbc/0x710 fs/aio.c:603
>> but this lock took another, SOFTIRQ-unsafe lock in the past:
>>  (&fiq->waitq){+.+.}
>>
>>
>> and interrupts could create inverse lock ordering between them.
>>
>>
>> other info that might help us debug this:
>>  Possible interrupt unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(&fiq->waitq);
>>                                local_irq_disable();
>>                                lock(&(&ctx->ctx_lock)->rlock);
>>                                lock(&fiq->waitq);
>>   <Interrupt>
>>     lock(&(&ctx->ctx_lock)->rlock);
>
> Fuse device doesn't support AIO ops.  So false positive, AFAICS.

Hi Miklos,

We still need to annotate this. How?