vsock/virtio: account only unread bytes in read_skb()

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>
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Link: https://patch.msgid.link/20261001151310.4101006-1-4ncienth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions