linux-kernel - [PATCH AUTOSEL 4.9 2/3] vringh: Use wiov->used to check for read/write desc order

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Message-Id: <20210824005539.631820-2-sashal@kernel.org>
Date:   Mon, 23 Aug 2021 20:55:38 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Neeraj Upadhyay <neeraju@...eaurora.org>,
        Jason Wang <jasowang@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Sasha Levin <sashal@...nel.org>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: [PATCH AUTOSEL 4.9 2/3] vringh: Use wiov->used to check for read/write desc order

From: Neeraj Upadhyay <neeraju@...eaurora.org>

[ 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 <jasowang@...hat.com>
Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>
Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
Link: https://lore.kernel.org/r/1624591502-4827-1-git-send-email-neeraju@codeaurora.org
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 d56736655dec..da47542496cc 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -329,7 +329,7 @@ __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