KMSAN reported the following uninit-value access issue:

=====================================================
BUG: KMSAN: uninit-value in virtio_transport_recv_pkt+0x1dfb/0x26a0 net/vmw_vsock/virtio_transport_common.c:1421
 virtio_transport_recv_pkt+0x1dfb/0x26a0 net/vmw_vsock/virtio_transport_common.c:1421
 vsock_loopback_work+0x3bb/0x5a0 net/vmw_vsock/vsock_loopback.c:120
 process_one_work kernel/workqueue.c:2630 [inline]
 process_scheduled_works+0xff6/0x1e60 kernel/workqueue.c:2703
 worker_thread+0xeca/0x14d0 kernel/workqueue.c:2784
 kthread+0x3cc/0x520 kernel/kthread.c:388
 ret_from_fork+0x66/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:304

Uninit was stored to memory at:
 virtio_transport_space_update net/vmw_vsock/virtio_transport_common.c:1274 [inline]
 virtio_transport_recv_pkt+0x1ee8/0x26a0 net/vmw_vsock/virtio_transport_common.c:1415
 vsock_loopback_work+0x3bb/0x5a0 net/vmw_vsock/vsock_loopback.c:120
 process_one_work kernel/workqueue.c:2630 [inline]
 process_scheduled_works+0xff6/0x1e60 kernel/workqueue.c:2703
 worker_thread+0xeca/0x14d0 kernel/workqueue.c:2784
 kthread+0x3cc/0x520 kernel/kthread.c:388
 ret_from_fork+0x66/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:304

Uninit was created at:
 slab_post_alloc_hook+0x105/0xad0 mm/slab.h:767
 slab_alloc_node mm/slub.c:3478 [inline]
 kmem_cache_alloc_node+0x5a2/0xaf0 mm/slub.c:3523
 kmalloc_reserve+0x13c/0x4a0 net/core/skbuff.c:559
 __alloc_skb+0x2fd/0x770 net/core/skbuff.c:650
 alloc_skb include/linux/skbuff.h:1286 [inline]
 virtio_vsock_alloc_skb include/linux/virtio_vsock.h:66 [inline]
 virtio_transport_alloc_skb+0x90/0x11e0 net/vmw_vsock/virtio_transport_common.c:58
 virtio_transport_reset_no_sock net/vmw_vsock/virtio_transport_common.c:957 [inline]
 virtio_transport_recv_pkt+0x1279/0x26a0 net/vmw_vsock/virtio_transport_common.c:1387
 vsock_loopback_work+0x3bb/0x5a0 net/vmw_vsock/vsock_loopback.c:120
 process_one_work kernel/workqueue.c:2630 [inline]
 process_scheduled_works+0xff6/0x1e60 kernel/workqueue.c:2703
 worker_thread+0xeca/0x14d0 kernel/workqueue.c:2784
 kthread+0x3cc/0x520 kernel/kthread.c:388
 ret_from_fork+0x66/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:304

CPU: 1 PID: 10664 Comm: kworker/1:5 Not tainted 6.6.0-rc3-00146-g9f3ebbef #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-1.fc38 04/01/2014
Workqueue: vsock-loopback vsock_loopback_work
=====================================================

The following simple reproducer can cause the issue described above:

int main(void)
{
  int sock;
  struct sockaddr_vm addr = {
    .svm_family = AF_VSOCK,
    .svm_cid = VMADDR_CID_ANY,
    .svm_port = 1234,
  };

  sock = socket(AF_VSOCK, SOCK_STREAM, 0);
  connect(sock, (struct sockaddr *)&addr, sizeof(addr));
  return 0;
}

This issue occurs because the `buf_alloc` and `fwd_cnt` fields of the
`struct virtio_vsock_hdr` are not initialized when a new skb is allocated
in `virtio_transport_init_hdr()`. This patch resolves the issue by
initializing these fields during allocation.

Fixes: 71dc9ec9 ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
Reported-and-tested-by: syzbot+0c8ce1da0ac31abbadcd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=0c8ce1da0ac31abbadcdSigned-off-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20231104150531.257952-1-syoshida@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>