On Thu, 17 Jul 2025 at 11:10, Jason Wang [off-list ref] wrote:

On Thu, Jul 17, 2025 at 5:01 PM Will Deacon [off-list ref] wrote:

quoted

vhost_vsock_alloc_skb() returns NULL for packets advertising a length
larger than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE in the packet header. However,
this is only checked once the SKB has been allocated and, if the length
in the packet header is zero, the SKB may not be freed immediately.

Can this be triggered from the guest? (I guess yes) Did we need to
consider it as a security issue?

Yep, but then the packet would still be discarded later, and the
memory released, so it can only increase the pressure on allocation,
but the guest can still do so by sending packets for example on an
unopened port.

Stefano

quoted

Hoist the size check before the SKB allocation so that an iovec larger
than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE + the header size is rejected
outright. The subsequent check on the length field in the header can
then simply check that the allocated SKB is indeed large enough to hold
the packet.

Cc: <redacted>
Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
 drivers/vhost/vsock.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 802153e23073..66a0f060770e 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c

@@ -344,6 +344,9 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,

        len = iov_length(vq->iov, out);

+       if (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)

@@ -367,8 +370,7 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,
                return skb;

        /* The pkt is too big or the length in the header is invalid */
-       if (payload_len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE ||
-           payload_len + sizeof(*hdr) > len) {
+       if (payload_len + sizeof(*hdr) > len) {
                kfree_skb(skb);
                return NULL;
        }
--

2.50.0.727.gbf7dc18ff4-goog

Thanks