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

[Michal Luczaj <mhal@rbox.co>]

On 6/20/25 16:43, Stefano Garzarella wrote:
> On Fri, 20 Jun 2025 at 16:23, Michal Luczaj <mhal@rbox.co> wrote:
>>
>> 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@rbox.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.
> 
> It's friday... yep, you're right!
> 
>>
>>> 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()?
> 
> Should return the G2H, LOCAL is more for debug/test, so I'd return it
> only if anything else is loaded.
>>>> 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().
>>
> 
> Feel free to drop 1 too, we can send it later if it's not really
> related to this issue.

I've added it to the end of this series (and marked the series as RFC), for
ease of discussion.

> About the series, maybe it is better to have a single patch that fixes
> the access to ->get_local_cid() with proper locking.
> But I don't have a strong opinion on that. I see it like a single
> problem to fix, but up to you.

Yeah, I get your point. So I've tried something similar in v2:
https://lore.kernel.org/netdev/20250620-vsock-transports-toctou-v2-0-02ebd20b1d03@rbox.co/