Re: [PATCH net 1/3] vsock: Fix transport_{h2g,g2h} TOCTOU

[Stefano Garzarella]

On Fri, Jun 20, 2025 at 02:58:49PM +0200, Michal Luczaj wrote:
> On 6/20/25 10:32, Stefano Garzarella wrote:
> > On Wed, Jun 18, 2025 at 02:34:00PM +0200, Michal Luczaj wrote:
> > > Checking transport_{h2g,g2h} != NULL may race with vsock_core_unregister().
> > > Make sure pointers remain valid.
> > >
> > > KASAN: null-ptr-deref in range [0x0000000000000118-0x000000000000011f]
> > > RIP: 0010:vsock_dev_do_ioctl.isra.0+0x58/0xf0
> > > Call Trace:
> > > __x64_sys_ioctl+0x12d/0x190
> > > do_syscall_64+0x92/0x1c0
> > > entry_SYSCALL_64_after_hwframe+0x4b/0x53
> > >
> > > Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
> > > Signed-off-by: Michal Luczaj <mhal@xxxxxxx>
> > > ---
> > > net/vmw_vsock/af_vsock.c | 4 ++++
> > > 1 file changed, 4 insertions(+)
> > >
> > > diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> > > index 2e7a3034e965db30b6ee295370d866e6d8b1c341..047d1bc773fab9c315a6ccd383a451fa11fb703e 100644
> > > --- a/net/vmw_vsock/af_vsock.c
> > > +++ b/net/vmw_vsock/af_vsock.c
> > > @@ -2541,6 +2541,8 @@ static long vsock_dev_do_ioctl(struct file *filp,
> > >
> > > 	switch (cmd) {
> > > 	case IOCTL_VM_SOCKETS_GET_LOCAL_CID:
> > > +		mutex_lock(&vsock_register_mutex);
> > > +
> > > 		/* To be compatible with the VMCI behavior, we prioritize the
> > > 		 * guest CID instead of well-know host CID (VMADDR_CID_HOST).
> > > 		 */
> > > @@ -2549,6 +2551,8 @@ static long vsock_dev_do_ioctl(struct file *filp,
> > > 		else if (transport_h2g)
> > > 			cid = transport_h2g->get_local_cid();
> > >
> > > +		mutex_unlock(&vsock_register_mutex);
> >
> >
> > What about if we introduce a new `vsock_get_local_cid`:
> >
> > u32 vsock_get_local_cid() {
> > 	u32 cid = VMADDR_CID_ANY;
> >
> > 	mutex_lock(&vsock_register_mutex);
> > 	/* To be compatible with the VMCI behavior, we prioritize the
> > 	 * guest CID instead of well-know host CID (VMADDR_CID_HOST).
> > 	 */
> > 	if (transport_g2h)
> > 		cid = transport_g2h->get_local_cid();
> > 	else if (transport_h2g)
> > 		cid = transport_h2g->get_local_cid();
> > 	mutex_lock(&vsock_register_mutex);
> >
> > 	return cid;
> > }
> >
> >
> > And we use it here, and in the place fixed by next patch?
> >
> > I think we can fix all in a single patch, the problem here is to call
> > transport_*->get_local_cid() without the lock IIUC.
>
> Do you mean:
>
> bool vsock_find_cid(unsigned int cid)
> {
> -       if (transport_g2h && cid == transport_g2h->get_local_cid())
> +       if (transport_g2h && cid == vsock_get_local_cid())
>                return true;
>
> ?

Nope, I meant:

 bool vsock_find_cid(unsigned int cid)
 {
-       if (transport_g2h && cid == transport_g2h->get_local_cid())
-               return true;
-
-       if (transport_h2g && cid == VMADDR_CID_HOST)
+       if (cid == vsock_get_local_cid())
                return true;

        if (transport_local && cid == VMADDR_CID_LOCAL)

But now I'm thinking if we should also include `transport_local` in the 
new `vsock_get_local_cid()`.

I think that will fix an issue when calling 
IOCTL_VM_SOCKETS_GET_LOCAL_CID and only vsock-loopback kernel module is 
loaded, so maybe we can do 2 patches:

1. fix IOCTL_VM_SOCKETS_GET_LOCAL_CID to check also `transport_local`
   Fixes: 0e12190578d0 ("vsock: add local transport support in the vsock core")

2. move that code in vsock_get_local_cid() with proper locking and use 
it also in vsock_find_cid()

WDYT?

Thanks,
Stefano