virtio-blk: limit seg_max to a safe value

The struct virtio_blk_config seg_max value is read from the device and
incremented by 2 to account for the request header and status byte
descriptors added by the driver.

In preparation for supporting untrusted virtio-blk devices, protect
against integer overflow and limit the value to a safe maximum.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20210524154020.98195-1-stefanha@redhat.comReviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/block/virtio_blk.c

@@ -21,6 +21,9 @@
 #define VQ_NAME_LEN 16
 #define MAX_DISCARD_SEGMENTS 256u
 
+/* The maximum number of sg elements that fit into a virtqueue */
+#define VIRTIO_BLK_MAX_SG_ELEMS 32768
+
 static int major;
 static DEFINE_IDA(vd_index_ida);
 
@@ -721,7 +724,10 @@ static int virtblk_probe(struct virtio_device *vdev)
 	if (err || !sg_elems)
 		sg_elems = 1;
 
-	/* We need an extra sg elements at head and tail. */
+	/* Prevent integer overflows and honor max vq size */
+	sg_elems = min_t(u32, sg_elems, VIRTIO_BLK_MAX_SG_ELEMS - 2);
+
+	/* We need extra sg elements at head and tail. */
 	sg_elems += 2;
 	vdev->priv = vblk = kmalloc(sizeof(*vblk), GFP_KERNEL);
 	if (!vblk) {