On Thu, Oct 04, 2018 at 12:28:56PM -0700, Eric Dumazet wrote:

On Thu, Oct 4, 2018 at 11:59 AM Dmitry Torokhov
[off-list ref] wrote:

quoted

Hi Eric,

On Thu, Oct 04, 2018 at 08:47:49AM -0700, Eric Dumazet wrote:

quoted

syzbot was able to trigger rcu stalls by calling write()
with large number of bytes.

Add a cond_resched() in the loop to avoid this.

I think this simply masks a deeper issue. The code fetches characters
from userspace in a loop, takes a lock, quickly places response in an
output buffer, and releases interrupt. I do not see why this should
cause stalls as we do not hold spinlock/interrupts off for extended
period of time.

Adding Paul so he can straighten me out...

Well...

write(fd, buffer, 0x7FFF0000);

Takes between 20 seconds and 2 minutes depending on CONFIG options ....

And two minutes would get you an RCU CPU stall warning, even on distro
kernels that set the stall-warning time to a full minute (as opposed
to 21 seconds in mainline).

So either apply my patch, or add a limit on the max count, and
possibly break legitimate user space ?

I dunno...

I vote for Eric's patch.  In fact:

Reviewed-by: Paul E. McKenney <redacted>

quoted

quoted

Link: https://lkml.org/lkml/2018/8/23/1106
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot+9436b02171ac0894d33e@syzkaller.appspotmail.com
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
---
 drivers/input/mousedev.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/input/mousedev.c b/drivers/input/mousedev.c
index e08228061bcdd2f97aaadece31d6c83eb7539ae5..412fa71245afe26a7a8ad75705566f83633ba347 100644
--- a/drivers/input/mousedev.c
+++ b/drivers/input/mousedev.c

@@ -707,6 +707,7 @@ static ssize_t mousedev_write(struct file *file, const char __user *buffer,
              mousedev_generate_response(client, c);

              spin_unlock_irq(&client->packet_lock);
+             cond_resched();
      }

      kill_fasync(&client->fasync, SIGIO, POLL_IN);
--

2.19.0.605.g01d371f741-goog

Thanks.

--
Dmitry