fuzz: virtio: Add fuzzer for vring

Add a fuzzer to test the vring handling code against unexpected
mutations from the virtio device.

After building the sandbox with CONFIG_FUZZ=y, the fuzzer can be invoked
with by:

   UBOOT_SB_FUZZ_TEST=fuzz_vring ./u-boot

This fuzzer finds unvalidated inputs in the vring driver that allow a
buggy or malicious device to make the driver chase wild pointers.

Signed-off-by: Andrew Scull <ascull@google.com>

2 files changed, 73 insertions, 0 deletions

diff --git a/test/fuzz/Makefile b/test/fuzz/Makefile
index 03eeeeb497..663b79ce80 100644
--- a/test/fuzz/Makefile
+++ b/test/fuzz/Makefile
@@ -5,3 +5,4 @@
 #
 
 obj-$(CONFIG_$(SPL_)CMDLINE) += cmd_fuzz.o
+obj-$(CONFIG_VIRTIO_SANDBOX) += virtio.o
diff --git a/test/fuzz/virtio.c b/test/fuzz/virtio.c
new file mode 100644
index 0000000000..e5363d5638
--- /dev/null
+++ b/test/fuzz/virtio.c
@@ -0,0 +1,72 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c) 2022 Google, Inc.
+ * Written by Andrew Scull <ascull@google.com>
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <virtio.h>
+#include <virtio_ring.h>
+#include <test/fuzz.h>
+
+static int fuzz_vring(const uint8_t *data, size_t size)
+{
+	struct udevice *bus, *dev;
+	struct virtio_dev_priv *uc_priv;
+	struct virtqueue *vq;
+	struct virtio_sg sg[2];
+	struct virtio_sg *sgs[2];
+	unsigned int len;
+	u8 buffer[2][32];
+
+	/* hackily hardcode vring sizes */
+	size_t num = 4;
+	size_t desc_size = (sizeof(struct vring_desc) * num);
+	size_t avail_size = (3 + num) * sizeof(u16);
+	size_t used_size = (3 * sizeof(u16)) + (sizeof(struct vring_used_elem) * num);
+
+	if (size < (desc_size + avail_size + used_size))
+		return 0;
+
+	/* check probe success */
+	if (uclass_first_device(UCLASS_VIRTIO, &bus) || !bus)
+		panic("Could not find virtio bus\n");
+
+	/* check the child virtio-rng device is bound */
+	if (device_find_first_child(bus, &dev) || !dev)
+		panic("Could not find virtio device\n");
+
+	/*
+	 * fake the virtio device probe by filling in uc_priv->vdev
+	 * which is used by virtio_find_vqs/virtio_del_vqs.
+	 */
+	uc_priv = dev_get_uclass_priv(bus);
+	uc_priv->vdev = dev;
+
+	/* prepare the scatter-gather buffer */
+	sg[0].addr = buffer[0];
+	sg[0].length = sizeof(buffer[0]);
+	sg[1].addr = buffer[1];
+	sg[1].length = sizeof(buffer[1]);
+	sgs[0] = &sg[0];
+	sgs[1] = &sg[1];
+
+	if (virtio_find_vqs(dev, 1, &vq))
+		panic("Could not find vqs\n");
+	if (virtqueue_add(vq, sgs, 0, 1))
+		panic("Could not add to virtqueue\n");
+	/* Simulate device writing to vring */
+	memcpy(vq->vring.desc, data, desc_size);
+	memcpy(vq->vring.avail, data + desc_size, avail_size);
+	memcpy(vq->vring.used, data + desc_size + avail_size, used_size);
+	/* Make sure there is a response */
+	if (vq->vring.used->idx == 0)
+		vq->vring.used->idx = 1;
+	virtqueue_get_buf(vq, &len);
+	if (virtio_del_vqs(dev))
+		panic("Could not delete vqs\n");
+
+	return 0;
+}
+FUZZ_TEST(fuzz_vring, 0);