Re: [PATCH v1 5/6] virtio_net: Guard against buffer length overflow in xdp_linearize_page()

When reassembling incoming buffers to an xdp_page, there is a potential
integer overflow in the buffer size test and trigger and out of bounds
memcpy().

Fix this by reordering the test so that both sides are of the same
signedness.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jesper Dangaard Brouer <hawk@kernel.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
---
 drivers/net/virtio_net.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 7723b2a49d8e..dfa51dd95f63 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -751,8 +751,10 @@ static struct page *xdp_linearize_page(struct receive_queue *rq,
 
 		/* guard against a misconfigured or uncooperative backend that
 		 * is sending packet larger than the MTU.
+		 * At the same time, make sure that an especially uncooperative
+		 * backend can't overflow the test by supplying a large buflen.
 		 */
-		if ((page_off + buflen + tailroom) > PAGE_SIZE) {
+		if (buflen > PAGE_SIZE - page_off - tailroom) {
 			put_page(p);
 			goto err_buf;
 		}