linux-kernel - Re: [PATCH net 1/3] vsock: Fix transport

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Message-ID: <4f0e2cc5-f3a0-4458-9954-438911e7d104@rbox.co>
Date: Fri, 20 Jun 2025 16:23:10 +0200
From: Michal Luczaj <mhal@...x.co>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 virtualization@...ts.linux.dev, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/3] vsock: Fix transport_{h2g,g2h} TOCTOU

On 6/20/25 15:20, Stefano Garzarella wrote:
> 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@...x.co>
>>>> ---
>>>> 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 it does change the behaviour, doesn't it? With this patch, (with g2h
loaded) if cid fails to match g2h->get_local_cid(), we don't fall back to
h2g case any more, i.e. no more comparing cid with VMADDR_CID_HOST.

> 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")

What would be the transport priority with transport_local thrown in? E.g.
if we have both local and g2h, ioctl should return VMADDR_CID_LOCAL or
transport_g2h->get_local_cid()?

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

Yeah, sure about 1, I'll add it to the series. I'm just still not certain
how useful vsock_get_local_cid() would be for vsock_find_cid().