[PATCH 6.12 024/169] vsock/virtio: Coalesce only linear skb

Greg Kroah-Hartman
2026-01-28T15:41:51Z

From: Michal Luczaj <mhal@rbox.co>

[ Upstream commit 0386bd321d0f95d041a7b3d7b07643411b044a96 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/vmw_vsock/virtio_transport_common.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 2c9b1011cdcc8..4e8a9771a04d6 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1374,9 +1374,11 @@ 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;

--
2.51.0