Re: [PATCH v3] virtio-rng: return available data with O_NONBLOCK

[Martin Wilck]

On Wed, 2020-08-26 at 08:26 -0400, Michael S. Tsirkin wrote:
> On Tue, Aug 11, 2020 at 04:42:32PM +0200, Laurent Vivier wrote:
> > On 11/08/2020 16:28, mwilck@suse.com wrote:
> > > From: Martin Wilck <mwilck@suse.com>
> > > 
> > > If a program opens /dev/hwrng with O_NONBLOCK and uses poll() and
> > > non-blocking read() to retrieve random data, it ends up in a
> > > tight
> > > loop with poll() always returning POLLIN and read() returning
> > > EAGAIN.
> > > This repeats forever until some process makes a blocking read()
> > > call.
> > > The reason is that virtio_read() always returns 0 in non-blocking 
> > > mode,
> > > even if data is available. Worse, it fetches random data from the
> > > hypervisor after every non-blocking call, without ever using this
> > > data.
> > > 
> > > The following test program illustrates the behavior and can be
> > > used
> > > for testing and experiments. The problem will only be seen if all
> > > tasks use non-blocking access; otherwise the blocking reads will
> > > "recharge" the random pool and cause other, non-blocking reads to
> > > succeed at least sometimes.
> > > 
> > > /* Whether to use non-blocking mode in a task, problem occurs if
> > > CONDITION is 1 */
> > > //#define CONDITION (getpid() % 2 != 0)
> > > 
> > > static volatile sig_atomic_t stop;
> > > static void handler(int sig __attribute__((unused))) { stop = 1;
> > > }
> > > 
> > > static void loop(int fd, int sec)
> > > {
> > >   struct pollfd pfd = { .fd = fd, .events  = POLLIN, };
> > >   unsigned long errors = 0, eagains = 0, bytes = 0, succ = 0;
> > >   int size, rc, rd;
> > > 
> > >   srandom(getpid());
> > >   if (CONDITION && fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) |
> > > O_NONBLOCK) == -1)
> > >           perror("fcntl");
> > >   size = MINBUFSIZ + random() % (MAXBUFSIZ - MINBUFSIZ + 1);
> > > 
> > >   for(;;) {
> > >           char buf[size];
> > > 
> > >           if (stop)
> > >                   break;
> > >           rc = poll(&pfd, 1, sec);
> > >           if (rc > 0) {
> > >                   rd = read(fd, buf, sizeof(buf));
> > >                   if (rd == -1 && errno == EAGAIN)
> > >                           eagains++;
> > >                   else if (rd == -1)
> > >                           errors++;
> > >                   else {
> > >                           succ++;
> > >                           bytes += rd;
> > >                           write(1, buf, sizeof(buf));
> > >                   }
> > >           } else if (rc == -1) {
> > >                   if (errno != EINTR)
> > >                           perror("poll");
> > >                   break;
> > >           } else
> > >                   fprintf(stderr, "poll: timeout\n");
> > >   }
> > >   fprintf(stderr,
> > >           "pid %d %sblocking, bufsize %d, %d seconds, %lu bytes
> > > read, %lu success, %lu eagain, %lu errors\n",
> > >           getpid(), CONDITION ? "non-" : "", size, sec, bytes,
> > > succ, eagains, errors);
> > > }
> > > 
> > > int main(void)
> > > {
> > >   int fd;
> > > 
> > >   fork(); fork();
> > >   fd = open("/dev/hwrng", O_RDONLY);
> > >   if (fd == -1) {
> > >           perror("open");
> > >           return 1;
> > >   };
> > >   signal(SIGALRM, handler);
> > >   alarm(SECONDS);
> > >   loop(fd, SECONDS);
> > >   close(fd);
> > >   wait(NULL);
> > >   return 0;
> > > }
> > > 
> > > void loop(int fd)
> > > {
> > >         struct pollfd pfd0 = { .fd = fd, .events  = POLLIN, };
> > >         int rc;
> > >         unsigned int n;
> > > 
> > >         for (n = LOOPS; n > 0; n--) {
> > >                 struct pollfd pfd = pfd0;
> > >                 char buf[SIZE];
> > > 
> > >                 rc = poll(&pfd, 1, 1);
> > >                 if (rc > 0) {
> > >                         int rd = read(fd, buf, sizeof(buf));
> > > 
> > >                         if (rd == -1)
> > >                                 perror("read");
> > >                         else
> > >                                 printf("read %d bytes\n", rd);
> > >                 } else if (rc == -1)
> > >                         perror("poll");
> > >                 else
> > >                         fprintf(stderr, "timeout\n");
> > > 
> > >         }
> > > }
> > > 
> > > int main(void)
> > > {
> > >         int fd;
> > > 
> > >         fd = open("/dev/hwrng", O_RDONLY|O_NONBLOCK);
> > >         if (fd == -1) {
> > >                 perror("open");
> > >                 return 1;
> > >         };
> > >         loop(fd);
> > >         close(fd);
> > >         return 0;
> > > }
> > > 
> > > This can be observed in the real word e.g. with nested qemu/KVM
> > > virtual
> > > machines, if both the "outer" and "inner" VMs have a virtio-rng
> > > device.
> > > If the "inner" VM requests random data, qemu running in the
> > > "outer" VM
> > > uses this device in a non-blocking manner like the test program
> > > above.
> > > 
> > > Fix it by returning available data if a previous hypervisor call
> > > has
> > > completed. I tested this patch with the program above, and with
> > > rng-tools.
> > > 
> > > v2 -> v3: Simplified the implementation as suggested by Laurent
> > > Vivier
> > > 
> > > Signed-off-by: Martin Wilck <mwilck@suse.com>
> > > ---
> > >  drivers/char/hw_random/virtio-rng.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/char/hw_random/virtio-rng.c
> > > b/drivers/char/hw_random/virtio-rng.c
> > > index a90001e02bf7..8eaeceecb41e 100644
> > > --- a/drivers/char/hw_random/virtio-rng.c
> > > +++ b/drivers/char/hw_random/virtio-rng.c
> > > @@ -65,7 +65,7 @@ static int virtio_read(struct hwrng *rng, void
> > > *buf, size_t size, bool wait)
> > >           register_buffer(vi, buf, size);
> > >   }
> > >  
> > > - if (!wait)
> > > + if (!wait && !completion_done(&vi->have_data))
> > >           return 0;
> > >  
> > >   ret = wait_for_completion_killable(&vi->have_data);
> > > @@ -74,7 +74,7 @@ static int virtio_read(struct hwrng *rng, void
> > > *buf, size_t size, bool wait)
> > >  
> > >   vi->busy = false;
> > >  
> > > - return vi->data_avail;
> > > + return min_t(size_t, size, vi->data_avail);
> > >  }
> > >  
> > >  static void virtio_cleanup(struct hwrng *rng)
> > > 
> > 
> > Reviewed-by: Laurent Vivier <lvivier@redhat.com>
> 
> Laurent didn't we agree the real fix is private buffers in the
> driver,
> and copying out from there?
> 

Can we perhaps proceed with this for now? AFAICS the private buffer
implementation would be a larger effort, while we have the issues with
nested VMs getting no entropy today.

Thanks
Martin