[PATCH 4.19 157/187] hwrng: virtio - Avoid repeated init of completion

From: Greg Kroah-Hartman
Date: Thu Apr 04 2019 - 05:05:59 EST

4.19-stable review patch. If anyone has any objections, please let me know.


[ Upstream commit aef027db48da56b6f25d0e54c07c8401ada6ce21 ]

The virtio-rng driver uses a completion called have_data to wait for a
virtio read to be fulfilled by the hypervisor. The completion is reset
before placing a buffer on the virtio queue and completed by the virtio
callback once data has been written into the buffer.

Prior to this commit, the driver called init_completion on this
completion both during probe as well as when registering virtio buffers
as part of a hwrng read operation. The second of these init_completion
calls should instead be reinit_completion because the have_data
completion has already been inited by probe. As described in
Documentation/scheduler/completion.txt, "Calling init_completion() twice
on the same completion object is most likely a bug".

This bug was present in the initial implementation of virtio-rng in
f7f510ec1957 ("virtio: An entropy device, as suggested by hpa"). Back
then the have_data completion was a single static completion rather than
a member of one of potentially multiple virtrng_info structs as
implemented later by 08e53fbdb85c ("virtio-rng: support multiple
virtio-rng devices"). The original driver incorrectly used
init_completion rather than INIT_COMPLETION to reset have_data during

Tested by running `head -c48 /dev/random | hexdump` within crosvm, the
Chrome OS virtual machine monitor, and confirming that the virtio-rng
driver successfully produces random bytes from the host.

Signed-off-by: David Tolnay <dtolnay@xxxxxxxxx>
Tested-by: David Tolnay <dtolnay@xxxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
drivers/char/hw_random/virtio-rng.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index b89df66ea1ae..7abd604e938c 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -73,7 +73,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)

if (!vi->busy) {
vi->busy = true;
- init_completion(&vi->have_data);
+ reinit_completion(&vi->have_data);
register_buffer(vi, buf, size);