[PATCH v2 2/3] vdpa/mlx5: should verify CTRL_VQ feature exists for MQ

[Si-Wei Liu <si-wei.liu@oracle.com>]

Per VIRTIO v1.1 specification, section 5.1.3.1 Feature bit requirements:
"VIRTIO_NET_F_MQ Requires VIRTIO_NET_F_CTRL_VQ".

There's assumption in the mlx5_vdpa multiqueue code that MQ must come
together with CTRL_VQ. However, there's nowhere in the upper layer to
guarantee this assumption would hold. Were there an untrusted driver
sending down MQ without CTRL_VQ, it would compromise various spots for
e.g. is_index_valid() and is_ctrl_vq_idx(). Although this doesn't end
up with immediate panic or security loophole as of today's code, the
chance for this to be taken advantage of due to future code change is
not zero.

Harden the crispy assumption by failing the set_driver_features() call
when seeing (MQ && !CTRL_VQ). For that end, verify_min_features() is
renamed to verify_driver_features() to reflect the fact that it now does
more than just validate the minimum features. verify_driver_features()
is now used to accommodate various checks against the driver features
for set_driver_features().

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
---
 drivers/vdpa/mlx5/net/mlx5_vnet.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index b53603d..6fa968f 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -1897,11 +1897,25 @@ static u64 mlx5_vdpa_get_device_features(struct vdpa_device *vdev)
 	return ndev->mvdev.mlx_features;
 }
 
-static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
+static int verify_driver_features(struct mlx5_vdpa_dev *mvdev, u64 features)
 {
+	/* Minimum features to expect */
 	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
 		return -EOPNOTSUPP;
 
+	/* Double check features combination sent down by the driver.
+	 * Fail invalid features due to absence of the depended feature.
+	 *
+	 * Per VIRTIO v1.1 specification, section 5.1.3.1 Feature bit
+	 * requirements: "VIRTIO_NET_F_MQ Requires VIRTIO_NET_F_CTRL_VQ".
+	 * By failing the invalid features sent down by untrusted drivers,
+	 * we're assured the assumption made upon is_index_valid() and
+	 * is_ctrl_vq_idx() will not be compromised.
+	 */
+	if ((features & (BIT_ULL(VIRTIO_NET_F_MQ) | BIT_ULL(VIRTIO_NET_F_CTRL_VQ))) ==
+            BIT_ULL(VIRTIO_NET_F_MQ))
+		return -EINVAL;
+
 	return 0;
 }
 
@@ -1977,7 +1991,7 @@ static int mlx5_vdpa_set_driver_features(struct vdpa_device *vdev, u64 features)
 
 	print_features(mvdev, features, true);
 
-	err = verify_min_features(mvdev, features);
+	err = verify_driver_features(mvdev, features);
 	if (err)
 		return err;
 
-- 
1.8.3.1

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