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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 31, 2020 at 02:37:26PM +0200, Laurent Vivier wrote:
> On 28/08/2020 23:34, Martin Wilck wrote:
> > 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.
> > 
> 
> I agree. I think it's important to have a simple and quick fix for the
> problem reported by Martin.
> 
> We need the private buffers but not sure how long it will take to have
> them included in the kernel and how many new bugs will be introduced
> doing that as the code is hard to understand and the core is shared with
> several other hardware backends that can be impacted by the changes needed.
> 
> Thanks,
> Laurent

However I am not sure with the patch applies we never return
the same buffer to userspace twice, e.g. if one is
non blocking another blocking. Doing that would be a bug.

-- 
MST

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