[PATCH net v2] vsock/virtio: account only unread bytes in read_skb()

[PATCH net v2] vsock/virtio: account only unread bytes in read_skb()

[Daehyeon Ko]

After a partial stream receive, rx_bytes tracks the unread suffix while
buf_used and peer credit still cover the whole packet.
virtio_transport_read_skb() dequeues that packet but passes pkt_len for
both counters, underflowing rx_bytes.

On a connected CID_LOCAL stream with an SK_SKB verdict, a 40-byte read
from a 100-byte packet made SIOCINQ report -40, an empty recv return
ELOOP, and poll report the empty socket readable.  After verdict detach,
receiving 40 bytes wrapped the counter to zero and hid those queued bytes
until one more byte arrived.  The fixed run kept the counter balanced
and exposed all queued data immediately.

This was found during an LLM-assisted manual source audit of VSOCK
receive accounting while re-evaluating virtio_transport_read_skb() after
CVE-2024-50169.

Subtract only skb->len minus the VSOCK offset from rx_bytes.  Retain the
full packet length for buf_used and peer credit.

Fixes: 45ca7e9f0730 ("vsock/virtio: fix `rx_bytes` accounting for stream sockets")
Cc: stable@vger.kernel.org
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Changes in v2:
- Narrow the subject prefix to vsock/virtio as suggested by Luigi Leonardi.
- State that the issue was found during an LLM-assisted manual source audit.
- Add Luigi's Reviewed-by tag.
- No code changes.

Link: https://lore.kernel.org/r/20261001121541.2983379-1-4ncienth@gmail.com
---
 net/vmw_vsock/virtio_transport_common.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index f225f53ed4bab..1e762a480df49 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1927,6 +1927,7 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
 	struct sock *sk = sk_vsock(vsk);
 	struct virtio_vsock_hdr *hdr;
 	struct sk_buff *skb;
+	u32 bytes_read;
 	u32 pkt_len;
 	int off = 0;
 	int err;
@@ -1946,7 +1947,8 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
 		vvs->msg_count--;
 
 	pkt_len = le32_to_cpu(hdr->len);
-	virtio_transport_dec_rx_pkt(vvs, pkt_len, pkt_len);
+	bytes_read = skb->len - VIRTIO_VSOCK_SKB_CB(skb)->offset;
+	virtio_transport_dec_rx_pkt(vvs, bytes_read, pkt_len);
 	spin_unlock_bh(&vvs->rx_lock);
 
 	virtio_transport_send_credit_update(vsk);

base-commit: e23a64eb244356ee47c0620f0722d51bd88db522
-- 
2.55.0