[PATCH 5.10 060/103] vringh: Use wiov->used to check for read/write desc order

From: Greg Kroah-Hartman
Date: Wed Sep 01 2021 - 08:42:09 EST

From: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>

[ Upstream commit e74cfa91f42c50f7f649b0eca46aa049754ccdbd ]

As __vringh_iov() traverses a descriptor chain, it populates
each descriptor entry into either read or write vring iov
and increments that iov's ->used member. So, as we iterate
over a descriptor chain, at any point, (riov/wriov)->used
value gives the number of descriptor enteries available,
which are to be read or written by the device. As all read
iovs must precede the write iovs, wiov->used should be zero
when we are traversing a read descriptor. Current code checks
for wiov->i, to figure out whether any previous entry in the
current descriptor chain was a write descriptor. However,
iov->i is only incremented, when these vring iovs are consumed,
at a later point, and remain 0 in __vringh_iov(). So, correct
the check for read and write descriptor order, to use

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/1624591502-4827-1-git-send-email-neeraju@xxxxxxxxxxxxxx
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
drivers/vhost/vringh.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index b7403ba8e7f7..0bd7e64331f0 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -341,7 +341,7 @@ __vringh_iov(struct vringh *vrh, u16 i,
iov = wiov;
else {
iov = riov;
- if (unlikely(wiov && wiov->i)) {
+ if (unlikely(wiov && wiov->used)) {
vringh_bad("Readable desc %p after writable",
err = -EINVAL;