vhost: fix hung thread due to erroneous iotlb entries

In vhost_iotlb_add_range_ctx(), range size can overflow to 0 when
start is 0 and last is ULONG_MAX. One instance where it can happen
is when userspace sends an IOTLB message with iova=size=uaddr=0
(vhost_process_iotlb_msg). So, an entry with size = 0, start = 0,
last = ULONG_MAX ends up in the iotlb. Next time a packet is sent,
iotlb_access_ok() loops indefinitely due to that erroneous entry.

	Call Trace:
	 <TASK>
	 iotlb_access_ok+0x21b/0x3e0 drivers/vhost/vhost.c:1340
	 vq_meta_prefetch+0xbc/0x280 drivers/vhost/vhost.c:1366
	 vhost_transport_do_send_pkt+0xe0/0xfd0 drivers/vhost/vsock.c:104
	 vhost_worker+0x23d/0x3d0 drivers/vhost/vhost.c:372
	 kthread+0x2e9/0x3a0 kernel/kthread.c:377
	 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
	 </TASK>

Reported by syzbot at:
	https://syzkaller.appspot.com/bug?extid=0abd373e2e50d704db87

To fix this, do two things:

1. Return -EINVAL in vhost_chr_write_iter() when userspace asks to map
   a range with size 0.
2. Fix vhost_iotlb_add_range_ctx() to handle the range [0, ULONG_MAX]
   by splitting it into two entries.

Fixes: 0bbe3066 ("vhost: factor out IOTLB")
Reported-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
Tested-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
Link: https://lore.kernel.org/r/20220305095525.5145-1-mail@anirudhrb.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/iotlb.c

@@ -57,6 +57,17 @@ int vhost_iotlb_add_range_ctx(struct vhost_iotlb *iotlb,
 	if (last < start)
 		return -EFAULT;
 
+	/* If the range being mapped is [0, ULONG_MAX], split it into two entries
+	 * otherwise its size would overflow u64.
+	 */
+	if (start == 0 && last == ULONG_MAX) {
+		u64 mid = last / 2;
+
+		vhost_iotlb_add_range_ctx(iotlb, start, mid, addr, perm, opaque);
+		addr += mid + 1;
+		start = mid + 1;
+	}
+
 	if (iotlb->limit &&
 	    iotlb->nmaps == iotlb->limit &&
 	    iotlb->flags & VHOST_IOTLB_FLAG_RETIRE) {

drivers/vhost/vhost.c

@@ -1170,6 +1170,11 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
 		goto done;
 	}
 
+	if (msg.size == 0) {
+		ret = -EINVAL;
+		goto done;
+	}
+
 	if (dev->msg_handler)
 		ret = dev->msg_handler(dev, &msg);
 	else