Subject: [PATCH AUTOSEL 5.13 09/26] vringh: Use wiov->used to check for read/write desc order

From: Sasha Levin <[email-masked]>
Date: Mon, 23 Aug 2021 20:53:39 -0400

From: Neeraj Upadhyay <[email-masked]>

[ 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
wiov->used.

Acked-by: Jason Wang <[email-masked]>
Reviewed-by: Stefano Garzarella <[email-masked]>
Signed-off-by: Neeraj Upadhyay <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Michael S. Tsirkin <[email-masked]>
Signed-off-by: Sasha Levin <[email-masked]>
---
 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 4af8fa259d65..14e2043d7685 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -359,7 +359,7 @@ __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",
 					   &descs[i]);
 				err = -EINVAL;
-- 
2.30.2