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

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Message-ID: <cg25zc7ktl6glh5r7mfxjvbjqguq2s2rj6vk24ful7zg6ydwuz@tjtvbrmemtpw>
Date: Fri, 20 Jun 2025 15:20:52 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Michal Luczaj <mhal@...x.co>
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 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 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