[PATCH net v2] vhost/vsock: size receive SKBs from declared payload

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

vhost_vsock_alloc_skb() allocates an skb from the total guest descriptor
length before reading hdr->len.  Descriptor capacity and declared payload
length are independent, so a guest can supply a large descriptor with a
zero or short payload.

On Linux v7.2, 455 zero-payload packets with 64 KiB descriptors retained
30,255,680 bytes on a 256 KiB receive buffer while rx_bytes and buf_used
stayed zero.  A full-payload control retained 265,984 bytes.  The existing
SKB_TRUESIZE(0) budget caps skb count but does not account for the
descriptor-sized allocation.

Repeating this across connections can exhaust host kernel memory.

Trimming after allocation is insufficient.  Linear skbs retain their full
head, while a nonlinear payload exceeding head tailroom can retain its
first page fragment.

Copy the header into a stack object, validate hdr->len before allocating,
and size the skb from the declared payload plus the header.  This keeps the
allocation proportional to receive accounting for both linear and
nonlinear skbs.

Use payload_len > len - sizeof(hdr) for validation.  This avoids addition
overflow on 32-bit hosts and ensures payload_len fits the subsequent
int-length copy path.

Fixes: ab9aa2f3afc2 ("vhost/vsock: Allocate nonlinear SKBs for handling large receive buffers")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Changes in v2:
- validate the header before allocation with an overflow-safe bounds check;
- size the skb from declared payload and remove the global trim-helper
  change;
- remove the generic clean-validation statement from the changelog; and
- add Will Deacon to Cc.

v1: https://lore.kernel.org/netdev/20260930044147.3818241-1-4ncienth@gmail.com/

Built and booted on v7.2 with KASAN, UBSAN and LOCKDEP.  A focused
allocation/queue probe retained 436,800 and 582,400 bytes for 455 zero and
341-byte packets, versus 30,255,680 bytes for the vulnerable zero case.  No
KASAN report, WARN, oops or panic occurred.  The probe injects skbs
in-kernel rather than through a live guest virtqueue.
 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