[PATCH] vhost: validate range size before adding to iotlb

[Anirudh Rayabharam]

In vhost_iotlb_add_range_ctx(), validate the range size is non-zero
before proceeding with adding it to the iotlb.

Range size can overflow to 0 when start is 0 and last is (2^64 - 1).
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 = (2^64 - 1) 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

Reported-by: syzbot+0abd373e2e50d704db87@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+0abd373e2e50d704db87@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Anirudh Rayabharam <mail@xxxxxxxxxxxxx>
---
 drivers/vhost/iotlb.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/iotlb.c b/drivers/vhost/iotlb.c
index 670d56c879e5..b9de74bd2f9c 100644
--- a/drivers/vhost/iotlb.c
+++ b/drivers/vhost/iotlb.c
@@ -53,8 +53,10 @@ int vhost_iotlb_add_range_ctx(struct vhost_iotlb *iotlb,
 			      void *opaque)
 {
 	struct vhost_iotlb_map *map;
+	u64 size = last - start + 1;
 
-	if (last < start)
+	// size can overflow to 0 when start is 0 and last is (2^64 - 1).
+	if (last < start || size == 0)
 		return -EFAULT;
 
 	if (iotlb->limit &&
@@ -69,7 +71,7 @@ int vhost_iotlb_add_range_ctx(struct vhost_iotlb *iotlb,
 		return -ENOMEM;
 
 	map->start = start;
-	map->size = last - start + 1;
+	map->size = size;
 	map->last = last;
 	map->addr = addr;
 	map->perm = perm;
-- 
2.35.1