Re: possible deadlock in free_ioctx_users

[Miklos Szeredi]

On Sun, Sep 9, 2018 at 8:41 PM, syzbot
<syzbot+d86c4426a01f60feddc7@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> 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.

Thanks,
Miklos