[PATCH v3] vhost/vdpa: reject VRING_NUM larger than device max

[Jia Jia <physicalmtea@gmail.com>]

vhost_vring_set_num() accepts any non-zero power-of-two queue size that
fits in 16 bits. vhost-vdpa then passes that value to set_vq_num()
without comparing it with get_vq_num_max().

A process with access to /dev/vhost-vdpa-* can therefore configure a
queue larger than the device advertises. With vdpa_sim, the worker can
walk 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

Cache get_vq_num_max() immediately after reset. Some backends derive
it from writable queue-size state, so querying it after SET_NUM may
return the current size instead of the device capability. Invalidate
the cached value before reset so a failed reset leaves SET_NUM
disabled.

For VHOST_SET_VRING_NUM, copy the complete vring state once and use
the same index and size for validation, vq->num, and set_vq_num().
This ensures that validation and use operate on the same copied values.

Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
Signed-off-by: Jia Jia <physicalmtea@gmail.com>
---
v3:
- Copy VHOST_SET_VRING_NUM once and use the copied values throughout,
  avoiding the double-fetch race in v2.
- Cache the device-wide maximum after reset so mutable queue state
  cannot break a later legal resize; fail closed if reset fails.

v2:
- Add the missing Signed-off-by tag.

 drivers/vhost/vdpa.c | 44 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 37 insertions(+), 7 deletions(-)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 692564b1bcbb..f2eedb0464c2 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -53,6 +53,7 @@ struct vhost_vdpa {
 	struct cdev cdev;
 	atomic_t opened;
 	u32 nvqs;
+	u16 vq_num_max;
 	int virtio_id;
 	int minor;
 	struct eventfd_ctx *config_ctx;
@@ -229,7 +230,9 @@ static void vhost_vdpa_unsetup_vq_irq(struct vhost_vdpa *v, u16 qid)
 static int _compat_vdpa_reset(struct vhost_vdpa *v)
 {
 	struct vdpa_device *vdpa = v->vdpa;
+	const struct vdpa_config_ops *ops = vdpa->config;
 	u32 flags = 0;
+	int ret;
 
 	v->suspended = false;
 
@@ -239,7 +242,14 @@ static int _compat_vdpa_reset(struct vhost_vdpa *v)
 			 VDPA_RESET_F_CLEAN_MAP : 0;
 	}
 
-	return vdpa_reset(vdpa, flags);
+	v->vq_num_max = 0;
+	ret = vdpa_reset(vdpa, flags);
+	if (!ret) {
+		/* Some backends derive the max from mutable queue state. */
+		v->vq_num_max = ops->get_vq_num_max(vdpa);
+	}
+
+	return ret;
 }
 
 static int vhost_vdpa_reset(struct vhost_vdpa *v)
@@ -641,9 +651,15 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
 	u32 idx;
 	long r;
 
-	r = get_user(idx, (u32 __user *)argp);
-	if (r < 0)
-		return r;
+	if (cmd == VHOST_SET_VRING_NUM) {
+		if (copy_from_user(&s, argp, sizeof(s)))
+			return -EFAULT;
+		idx = s.index;
+	} else {
+		r = get_user(idx, (u32 __user *)argp);
+		if (r < 0)
+			return r;
+	}
 
 	if (idx >= v->nvqs)
 		return -ENOBUFS;
@@ -652,6 +668,23 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
 	vq = &v->vqs[idx];
 
 	switch (cmd) {
+	case VHOST_SET_VRING_NUM:
+		mutex_lock(&vq->mutex);
+		if (vq->private_data) {
+			r = -EBUSY;
+		} else if (!s.num || s.num > 0xffff ||
+			   s.num > v->vq_num_max ||
+			   (s.num & (s.num - 1))) {
+			r = -EINVAL;
+		} else {
+			vq->num = s.num;
+			r = 0;
+		}
+		mutex_unlock(&vq->mutex);
+		if (r)
+			return r;
+		ops->set_vq_num(vdpa, idx, s.num);
+		return 0;
 	case VHOST_VDPA_SET_VRING_ENABLE:
 		if (copy_from_user(&s, argp, sizeof(s)))
 			return -EFAULT;
@@ -765,9 +798,6 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
 		ops->set_vq_cb(vdpa, idx, &cb);
 		break;
 
-	case VHOST_SET_VRING_NUM:
-		ops->set_vq_num(vdpa, idx, vq->num);
-		break;
 	}
 
 	return r;
-- 
2.34.1