[PATCH net v3] vhost/vsock: size receive SKBs from declared payload

[Daehyeon Ko <4ncienth@gmail.com>]

vhost_vsock_alloc_skb() sizes the skb from iov_length(), the total guest
descriptor length.  virtio_transport_inc_rx_pkt() instead adds hdr->len to
rx_bytes and buf_used, and budgets each queued skb as SKB_TRUESIZE(0).  The
guest controls the descriptor length and hdr->len independently.

A guest can therefore provide a 64 KiB descriptor while declaring a zero
payload.  The descriptor-sized skb remains queued, rx_bytes and buf_used
remain unchanged, and only the fixed queue-entry budget limits retention.
This is not a lifetime leak: the skb is freed when dequeued or at socket
teardown.  Repeating this across connections can nevertheless exhaust host
memory.  On Linux v7.2, 455 such descriptors retained 30,255,680 bytes for
a 262,144-byte receive buffer; a full-payload control retained 265,984
bytes.

Vhost previously read hdr->len and allocated pkt->buf from that value.  The
conversion from virtio_vsock_pkt to sk_buff instead allocated receive skbs
from the descriptor length, introducing the allocation-accounting mismatch.

Copy the header into a stack object and validate hdr->len before allocating
the skb.  Size the allocation as sizeof(hdr) + payload_len so descriptor
capacity no longer determines queued memory.  Check payload_len against
len - sizeof(hdr) to avoid addition overflow on 32-bit hosts.

Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Changes in v3:
- clarify that queued skb memory is under-accounted rather than leaked, and
  explain why a zero payload is the strongest case;
- remove v1-to-v2 implementation rationale from the permanent changelog;
- correct the Fixes tag from ab9aa2f3afc2 to 71dc9ec9ac7d; and
- make no code changes.

v2: https://lore.kernel.org/netdev/20261004073419.4039011-1-4ncienth@gmail.com/

The issue was identified during an LLM-assisted source audit and confirmed
with a focused in-kernel probe on Linux v7.2.  The code diff is
byte-identical to v2, so its existing build and runtime results remain
applicable.

 drivers/vhost/vsock.c | 34 ++++++++++++++++------------------
 1 file changed, 16 insertions(+), 18 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index abed1fbcf66cc5..35f75e23c7497e 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -364,7 +364,7 @@ static struct sk_buff *
 vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,
 		      unsigned int out, unsigned int in)
 {
-	struct virtio_vsock_hdr *hdr;
+	struct virtio_vsock_hdr hdr;
 	struct iov_iter iov_iter;
 	struct sk_buff *skb;
 	size_t payload_len;
@@ -382,34 +382,32 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,
 	    len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE + VIRTIO_VSOCK_SKB_HEADROOM)
 		return NULL;
 
-	/* len contains both payload and hdr */
-	skb = virtio_vsock_alloc_skb(len, GFP_KERNEL);
-	if (!skb)
-		return NULL;
-
 	iov_iter_init(&iov_iter, ITER_SOURCE, vq->iov, out, len);
 
-	hdr = virtio_vsock_hdr(skb);
-	nbytes = copy_from_iter(hdr, sizeof(*hdr), &iov_iter);
-	if (nbytes != sizeof(*hdr)) {
+	nbytes = copy_from_iter(&hdr, sizeof(hdr), &iov_iter);
+	if (nbytes != sizeof(hdr)) {
 		vq_err(vq, "Expected %zu bytes for pkt->hdr, got %zu bytes\n",
-		       sizeof(*hdr), nbytes);
-		kfree_skb(skb);
+		       sizeof(hdr), nbytes);
 		return NULL;
 	}
 
-	payload_len = le32_to_cpu(hdr->len);
+	payload_len = le32_to_cpu(hdr.len);
+
+	/* The pkt is too big or the length in the header is invalid */
+	if (payload_len > len - sizeof(hdr))
+		return NULL;
+
+	/* Allocate only for the payload declared in the header. */
+	skb = virtio_vsock_alloc_skb(payload_len + sizeof(hdr), GFP_KERNEL);
+	if (!skb)
+		return NULL;
+
+	memcpy(virtio_vsock_hdr(skb), &hdr, sizeof(hdr));
 
 	/* No payload */
 	if (!payload_len)
 		return skb;
 
-	/* The pkt is too big or the length in the header is invalid */
-	if (payload_len + sizeof(*hdr) > len) {
-		kfree_skb(skb);
-		return NULL;
-	}
-
 	virtio_vsock_skb_put(skb, payload_len);
 
 	if (skb_copy_datagram_from_iter(skb, 0, &iov_iter, payload_len)) {

base-commit: 6dc989ea46b96ce170840174b4a38c4a387fb005
-- 
2.55.0