[PATCH] vhost: handle zero regions in vhost_set_memory

[Anirudh Rayabharam]

Return early when userspace sends zero regions in the VHOST_SET_MEM_TABLE
ioctl.

Otherwise, this causes an erroneous entry to be added to the iotlb. This
entry has a range size of 0 (due to u64 overflow). This then causes
iotlb_access_ok() to loop indefinitely resulting in a hung thread.
Syzbot has reported this here:

https://syzkaller.appspot.com/bug?extid=0abd373e2e50d704db87

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

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 59edb5a1ffe2..821aba60eac2 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1428,6 +1428,8 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 		return -EFAULT;
 	if (mem.padding)
 		return -EOPNOTSUPP;
+	if (mem.nregions == 0)
+		return 0;
 	if (mem.nregions > max_mem_regions)
 		return -E2BIG;
 	newmem = kvzalloc(struct_size(newmem, regions, mem.nregions),
-- 
2.35.1