hwrng: virtio - don't waste entropy

[ Upstream commit 5c8e933050044d6dd2a000f9a5756ae73cbe7c44 ]

if we don't use all the entropy available in the buffer, keep it
and use it later.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Link: https://lore.kernel.org/r/20211028101111.128049-4-lvivier@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Stable-dep-of: ac52578d6e8d ("hwrng: virtio - Fix race on data_avail and actual data")
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index c8f5a3392e48c29154bbb2a078662e21aa19a55c..207a5f3b335c08941dd6a97253241cfff5e8cad5 100644 (file)
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -25,6 +25,7 @@ struct virtrng_info {
        /* data transfer */
        struct completion have_data;
        unsigned int data_avail;
+       unsigned int data_idx;
        /* minimal size returned by rng_buffer_size() */
 #if SMP_CACHE_BYTES < 32
        u8 data[32];
@@ -41,6 +42,9 @@ static void random_recv_done(struct virtqueue *vq)
        if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
                return;
 
+       vi->data_idx = 0;
+       vi->busy = false;
+
        complete(&vi->have_data);
 }
 
@@ -57,6 +61,16 @@ static void register_buffer(struct virtrng_info *vi)
        virtqueue_kick(vi->vq);
 }
 
+static unsigned int copy_data(struct virtrng_info *vi, void *buf,
+                             unsigned int size)
+{
+       size = min_t(unsigned int, size, vi->data_avail);
+       memcpy(buf, vi->data + vi->data_idx, size);
+       vi->data_idx += size;
+       vi->data_avail -= size;
+       return size;
+}
+
 static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 {
        int ret;
@@ -67,17 +81,29 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
        if (vi->hwrng_removed)
                return -ENODEV;
 
-       if (!vi->busy) {
-               vi->busy = true;
-               reinit_completion(&vi->have_data);
-               register_buffer(vi);
+       read = 0;
+
+       /* copy available data */
+       if (vi->data_avail) {
+               chunk = copy_data(vi, buf, size);
+               size -= chunk;
+               read += chunk;
        }
 
        if (!wait)
-               return 0;
+               return read;
 
-       read = 0;
+       /* We have already copied available entropy,
+        * so either size is 0 or data_avail is 0
+        */
        while (size != 0) {
+               /* data_avail is 0 */
+               if (!vi->busy) {
+                       /* no pending request, ask for more */
+                       vi->busy = true;
+                       reinit_completion(&vi->have_data);
+                       register_buffer(vi);
+               }
                ret = wait_for_completion_killable(&vi->have_data);
                if (ret < 0)
                        return ret;
@@ -87,20 +113,11 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
                if (vi->data_avail == 0)
                        return read;
 
-               chunk = min_t(unsigned int, size, vi->data_avail);
-               memcpy(buf + read, vi->data, chunk);
-               read += chunk;
+               chunk = copy_data(vi, buf + read, size);
                size -= chunk;
-               vi->data_avail = 0;
-
-               if (size != 0) {
-                       reinit_completion(&vi->have_data);
-                       register_buffer(vi);
-               }
+               read += chunk;
        }
 
-       vi->busy = false;
-
        return read;
 }
 
@@ -160,6 +177,7 @@ static void remove_common(struct virtio_device *vdev)
 
        vi->hwrng_removed = true;
        vi->data_avail = 0;
+       vi->data_idx = 0;
        complete(&vi->have_data);
        vdev->config->reset(vdev);
        vi->busy = false;