Linux Virtualization - Dec 2009 - [PATCH 1/3] vhost: prevent modification of an active ring

We don't support changing ring size or log base while ring is running.
If user violates these rules, he might get his memory silently
corrupted.  It's better to be explicit, and fail such modification
attempts with an error.
To make these "vq running" checks in ioctl context robust, this patch
also moves all vq flushes to under device mutex.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
 drivers/vhost/net.c   |    6 +++---
 drivers/vhost/vhost.c |   29 ++++++++++++++++++++++++-----
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index d6db10c..1b509a0 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -509,12 +509,10 @@ static long vhost_net_set_backend(struct vhost_net *n,
unsigned index, int fd)
 	vhost_net_enable_vq(n, vq);
 	mutex_unlock(&vq->mutex);
 done:
-	mutex_unlock(&n->dev.mutex);
 	if (oldsock) {
 		vhost_net_flush_vq(n, index);
 		fput(oldsock->file);
 	}
-	return r;
 err:
 	mutex_unlock(&n->dev.mutex);
 	return r;
@@ -554,8 +552,8 @@ static void vhost_net_set_features(struct vhost_net *n, u64
features)
 		n->vqs[i].hdr_size = hdr_size;
 		mutex_unlock(&n->vqs[i].mutex);
 	}
-	mutex_unlock(&n->dev.mutex);
 	vhost_net_flush(n);
+	mutex_unlock(&n->dev.mutex);
 }
 
 static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
@@ -587,8 +585,10 @@ static long vhost_net_ioctl(struct file *f, unsigned int
ioctl,
 	case VHOST_RESET_OWNER:
 		return vhost_net_reset_owner(n);
 	default:
+		mutex_lock(&n->dev.mutex);
 		r = vhost_dev_ioctl(&n->dev, ioctl, arg);
 		vhost_net_flush(n);
+		mutex_unlock(&n->dev.mutex);
 		return r;
 	}
 }
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index e7b4dea..29f1675 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -288,6 +288,12 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl,
void __user *argp)
 
 	switch (ioctl) {
 	case VHOST_SET_VRING_NUM:
+		/* Resizing ring with an active backend?
+		 * You don't want to do that. */
+		if (vq->private_data) {
+			r = -EBUSY;
+			break;
+		}
 		r = copy_from_user(&s, argp, sizeof s);
 		if (r < 0)
 			break;
@@ -298,6 +304,12 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl,
void __user *argp)
 		vq->num = s.num;
 		break;
 	case VHOST_SET_VRING_BASE:
+		/* Moving base with an active backend?
+		 * You don't want to do that. */
+		if (vq->private_data) {
+			r = -EBUSY;
+			break;
+		}
 		r = copy_from_user(&s, argp, sizeof s);
 		if (r < 0)
 			break;
@@ -413,6 +425,7 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl,
void __user *argp)
 	return r;
 }
 
+/* Caller must have device mutex */
 long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long
arg)
 {
 	void __user *argp = (void __user *)arg;
@@ -422,7 +435,6 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int
ioctl, unsigned long arg)
 	long r;
 	int i, fd;
 
-	mutex_lock(&d->mutex);
 	/* If you are not the owner, you can become one */
 	if (ioctl == VHOST_SET_OWNER) {
 		r = vhost_dev_set_owner(d);
@@ -447,9 +459,17 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int
ioctl, unsigned long arg)
 			break;
 		}
 		for (i = 0; i < d->nvqs; ++i) {
-			mutex_lock(&d->vqs[i].mutex);
-			d->vqs[i].log_base = (void __user *)(unsigned long)p;
-			mutex_unlock(&d->vqs[i].mutex);
+			struct vhost_virtqueue *vq;
+			vq = d->vqs + i;
+			mutex_lock(&vq->mutex);
+			/* Moving log base with an active backend?
+			 * You don't want to do that. */
+			if (vq->private_data && (vq->log_used ||
+			     vhost_has_feature(d, VHOST_F_LOG_ALL)))
+				r = -EBUSY;
+			else
+				vq->log_base = (void __user *)(unsigned long)p;
+			mutex_unlock(&vq->mutex);
 		}
 		break;
 	case VHOST_SET_LOG_FD:
@@ -483,7 +503,6 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int
ioctl, unsigned long arg)
 		break;
 	}
 done:
-	mutex_unlock(&d->mutex);
 	return r;
 }
 
-- 
1.6.6.rc1.43.gf55cc