[PATCH net v2] vsock/virtio: fix skb overhead accounting to preserve full buf_alloc

[Stefano Garzarella <sgarzare@redhat.com>]

From: Stefano Garzarella <sgarzare@redhat.com>

After commit 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb
queue"), virtio_transport_inc_rx_pkt() subtracts per-skb overhead from
buf_alloc when checking whether a new packet fits. This reduces the
effective receive buffer below what the user configured via
SO_VM_SOCKETS_BUFFER_SIZE, causing legitimate data packets to be
silently dropped and applications that rely on the full buffer size
to deadlock.

Also, the reduced space is not communicated to the remote peer, so
its credit calculation accounts more credit than the receiver will
actually accept, causing data loss (there is no retransmission).

With this approach we currently have failures in
tools/testing/vsock/vsock_test.c. Test 18 sometimes fails, while
test 22 always fails in this way:
    18 - SOCK_STREAM MSG_ZEROCOPY...hash mismatch

    22 - SOCK_STREAM virtio credit update + SO_RCVLOWAT...send failed:
    Resource temporarily unavailable

Fix this by using `buf_alloc * 2` as the total budget for payload plus
skb overhead in virtio_transport_inc_rx_pkt(), similar to how SO_RCVBUF
is doubled to reserve space for sk_buff metadata. This preserves the
full buf_alloc for payload under normal operation, while still bounding
the skb queue growth.

When the total budget (buf_alloc * 2) is exceeded (e.g. under small-packet
flooding where overhead dominates), the connection is reset and local
socket error set to ENOBUFS, so both peers are explicitly notified of
the failure rather than silently losing data.

With this patch, all tests in tools/testing/vsock/vsock_test.c are
now passing again.

A solution to handle small-packet overhead efficiently also for
SEQPACKET (we already do that for STREAM) is planned as follow-up work.
This patch is needed in any case to prevent silent data loss, because
even if we reduce the overhead, we can't eliminate it entirely.

Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
v2:
- Close the connection when we can no longer queue new packets instead
  of losing data.
- No longer announce the reduced buf_alloc to avoid violating the
  spec. [MST]

v1: https://lore.kernel.org/netdev/20260508092330.69690-1-sgarzare@redhat.com/
---
 net/vmw_vsock/virtio_transport_common.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 9b8014516f4f..f23bf8a11319 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -449,7 +449,10 @@ static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
 {
 	u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0);
 
-	if (skb_overhead + vvs->buf_used + len > vvs->buf_alloc)
+	/* Use buf_alloc * 2 as total budget (payload + overhead), similar to
+	 * how SO_RCVBUF is doubled to reserve space for sk_buff metadata.
+	 */
+	if (skb_overhead + vvs->buf_used + len > (u64)vvs->buf_alloc * 2)
 		return false;
 
 	vvs->rx_bytes += len;
@@ -1365,7 +1368,7 @@ virtio_transport_recv_connecting(struct sock *sk,
 	return err;
 }
 
-static void
+static bool
 virtio_transport_recv_enqueue(struct vsock_sock *vsk,
 			      struct sk_buff *skb)
 {
@@ -1380,10 +1383,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
 	spin_lock_bh(&vvs->rx_lock);
 
 	can_enqueue = virtio_transport_inc_rx_pkt(vvs, len);
-	if (!can_enqueue) {
-		free_pkt = true;
+	if (!can_enqueue)
 		goto out;
-	}
 
 	if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM)
 		vvs->msg_count++;
@@ -1423,6 +1424,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
 	spin_unlock_bh(&vvs->rx_lock);
 	if (free_pkt)
 		kfree_skb(skb);
+
+	return can_enqueue;
 }
 
 static int
@@ -1435,7 +1438,16 @@ virtio_transport_recv_connected(struct sock *sk,
 
 	switch (le16_to_cpu(hdr->op)) {
 	case VIRTIO_VSOCK_OP_RW:
-		virtio_transport_recv_enqueue(vsk, skb);
+		if (!virtio_transport_recv_enqueue(vsk, skb)) {
+			/* There is no more space to queue the packet, so let's
+			 * close the connection; otherwise, we'll lose data.
+			 */
+			(void)virtio_transport_reset(vsk, skb);
+			sk->sk_state = TCP_CLOSE;
+			sk->sk_err = ENOBUFS;
+			sk_error_report(sk);
+			break;
+		}
 		vsock_data_ready(sk);
 		return err;
 	case VIRTIO_VSOCK_OP_CREDIT_REQUEST:
-- 
2.54.0