vsock/virtio: Coalesce only linear skb

vsock/virtio common tries to coalesce buffers in rx queue: if a linear skb
(with a spare tail room) is followed by a small skb (length limited by
GOOD_COPY_LEN = 128), an attempt is made to join them.

Since the introduction of MSG_ZEROCOPY support, assumption that a small skb
will always be linear is incorrect. In the zerocopy case, data is lost and
the linear skb is appended with uninitialized kernel memory.

Of all 3 supported virtio-based transports, only loopback-transport is
affected. G2H virtio-transport rx queue operates on explicitly linear skbs;
see virtio_vsock_alloc_linear_skb() in virtio_vsock_rx_fill(). H2G
vhost-transport may allocate non-linear skbs, but only for sizes that are
not considered for coalescence; see PAGE_ALLOC_COSTLY_ORDER in
virtio_vsock_alloc_skb().

Ensure only linear skbs are coalesced. Note that skb_tailroom(last_skb) > 0
guarantees last_skb is linear.

Fixes: 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY flag support")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260113-vsock-recv-coalescence-v2-1-552b17837cf4@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index dcc8a1d5851e6a401dbc45bc980c363a37d1e1da..26b979ad71f09e9710900f94fead52c98e675673 100644 (file)
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1359,9 +1359,11 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
 
        /* Try to copy small packets into the buffer of last packet queued,
         * to avoid wasting memory queueing the entire buffer with a small
-        * payload.
+        * payload. Skip non-linear (e.g. zerocopy) skbs; these carry payload
+        * in skb_shinfo.
         */
-       if (len <= GOOD_COPY_LEN && !skb_queue_empty(&vvs->rx_queue)) {
+       if (len <= GOOD_COPY_LEN && !skb_queue_empty(&vvs->rx_queue) &&
+           !skb_is_nonlinear(skb)) {
                struct virtio_vsock_hdr *last_hdr;
                struct sk_buff *last_skb;