[PATCH net v3 2/2] 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.

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

Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 net/vmw_vsock/virtio_transport_common.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 4a4ac69d1ad1..e22117bf5dcd 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -434,7 +434,10 @@ 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;
-- 
2.54.0