FROMGIT: vhost/vsock: Avoid allocating arbitrarily-sized SKBs

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.

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: <stable@vger.kernel.org>
Fixes: 71dc9ec9 ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
Message-Id: <20250717090116.11987-2-will@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit c76f3c4364fe523cd2782269eab92529c86217aa
 git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git

 vhost)
Bug: 421244320
Bug: 436528862
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: Ie904fdc2790aadb12c9775e6ae3ca7a74fa0cc39