From: Herbert Xu herbert@gondor.apana.org.au
[ Upstream commit ac52578d6e8d300dd50f790f29a24169b1edd26c ]
The virtio rng device kicks off a new entropy request whenever the data available reaches zero. When a new request occurs at the end of a read operation, that is, when the result of that request is only needed by the next reader, then there is a race between the writing of the new data and the next reader.
This is because there is no synchronisation whatsoever between the writer and the reader.
Fix this by writing data_avail with smp_store_release and reading it with smp_load_acquire when we first enter read. The subsequent reads are safe because they're either protected by the first load acquire, or by the completion mechanism.
Also remove the redundant zeroing of data_idx in random_recv_done (data_idx must already be zero at this point) and data_avail in request_entropy (ditto).
Reported-by: syzbot+726dc8c62c3536431ceb@syzkaller.appspotmail.com Fixes: f7f510ec1957 ("virtio: An entropy device, as suggested by hpa.") Signed-off-by: Herbert Xu herbert@gondor.apana.org.au Acked-by: Michael S. Tsirkin mst@redhat.com Signed-off-by: Herbert Xu herbert@gondor.apana.org.au Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/char/hw_random/virtio-rng.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c index a6f3a8a2aca6d..35304117338ab 100644 --- a/drivers/char/hw_random/virtio-rng.c +++ b/drivers/char/hw_random/virtio-rng.c @@ -4,6 +4,7 @@ * Copyright (C) 2007, 2008 Rusty Russell IBM Corporation */
+#include <asm/barrier.h> #include <linux/err.h> #include <linux/hw_random.h> #include <linux/scatterlist.h> @@ -37,13 +38,13 @@ struct virtrng_info { static void random_recv_done(struct virtqueue *vq) { struct virtrng_info *vi = vq->vdev->priv; + unsigned int len;
/* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */ - if (!virtqueue_get_buf(vi->vq, &vi->data_avail)) + if (!virtqueue_get_buf(vi->vq, &len)) return;
- vi->data_idx = 0; - + smp_store_release(&vi->data_avail, len); complete(&vi->have_data); }
@@ -52,7 +53,6 @@ static void request_entropy(struct virtrng_info *vi) struct scatterlist sg;
reinit_completion(&vi->have_data); - vi->data_avail = 0; vi->data_idx = 0;
sg_init_one(&sg, vi->data, sizeof(vi->data)); @@ -88,7 +88,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait) read = 0;
/* copy available data */ - if (vi->data_avail) { + if (smp_load_acquire(&vi->data_avail)) { chunk = copy_data(vi, buf, size); size -= chunk; read += chunk;