[PATCH] vhost/vdpa: reject VRING_NUM larger than device max

[Jia Jia <physicalmtea@gmail.com>]

vhost_vring_set_num() only requires a non-zero power-of-two that fits
in 16 bits. vhost-vdpa then hands that value to the backend through
set_vq_num() without comparing it to get_vq_num_max().

A local process with access to /dev/vhost-vdpa-* can therefore set a
queue size larger than the device advertises. On the vdpa_sim backend,
the worker then walks descriptors beyond the mapped descriptor ring.
KASAN reports a 16-byte out-of-bounds read, corresponding to one
vring_desc, in the vringh IOTLB path:

  BUG: KASAN: out-of-bounds in _copy_from_iter
  Read of size 16
  copy_from_iotlb
  copydesc_iotlb
  vringh_getdesc_iotlb
  vdpasim_net_work

Check the requested size against get_vq_num_max() in
vhost_vdpa_vring_ioctl() before calling vhost_vring_ioctl(), and return
-EINVAL when the request is too large.

Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
---
 drivers/vhost/vdpa.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index ac55275fa0d0..b2083e1ef150 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -716,6 +716,13 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
 				vhost_vdpa_unsetup_vq_irq(v, idx);
 		}
 		break;
+	case VHOST_SET_VRING_NUM:
+		/* Reject sizes above the max reported by the device. */
+		if (copy_from_user(&s, argp, sizeof(s)))
+			return -EFAULT;
+		if (s.num > ops->get_vq_num_max(vdpa))
+			return -EINVAL;
+		break;
 	}
 
 	r = vhost_vring_ioctl(&v->vdev, cmd, argp);
-- 
2.34.1