On Thu, Oct 4, 2018 at 12:38 PM Dmitry Torokhov <dmitry.torok...@gmail.com> wrote: > > On October 4, 2018 12:28:56 PM PDT, Eric Dumazet <eduma...@google.com> wrote: > >On Thu, Oct 4, 2018 at 11:59 AM Dmitry Torokhov > ><dmitry.torok...@gmail.com> wrote: > >> > >> Hi Eric, > >> > >> On Thu, Oct 04, 2018 at 08:47:49AM -0700, Eric Dumazet wrote: > >> > 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 .... > > That's fine even if it takes a couple of years. We are not holding spinlock > for the entirety of this time, so we should get bumped off CPU at some point.
Well, you are saying that we could get rid of all cond_resched() calls in the kernel. You should send patches asap ;) > > > > >So either apply my patch, or add a limit on the max count, and > >possibly break legitimate user space ? > > Legitimate users write a single character at a time and read response, so > exciting after, let's say, 32 bytes would be fine. But I still want to > understand why we have to do that. > > > > >I dunno... > > > >> > > >> > Link: https://lkml.org/lkml/2018/8/23/1106 > >> > Signed-off-by: Eric Dumazet <eduma...@google.com> > >> > Reported-by: syzbot+9436b02171ac0894d...@syzkaller.appspotmail.com > >> > Cc: Dmitry Torokhov <dmitry.torok...@gmail.com> > >> > Cc: linux-in...@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 > > > Thanks. > > -- > Dmitry
