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vsock/virtio: Coalesce only linear skb

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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: 581512a6dc ("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>
This commit is contained in:
Michal Luczaj 2026-01-13 16:08:18 +01:00 committed by Jakub Kicinski
parent c4efd7a770
commit 0386bd321d
1 changed files with 4 additions and 2 deletions
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6
net/vmw_vsock/virtio_transport_common.c
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@ -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;