Re: [PATCH] net: tap: check if the file descriptor is valid before using it

[Laurent Vivier <lvivier@redhat.com>]

On 28/06/2020 08:31, Jason Wang wrote:
> 
> On 2020/6/25 下午7:56, Laurent Vivier wrote:
>> On 25/06/2020 10:48, Daniel P. Berrangé wrote:
>>> On Wed, Jun 24, 2020 at 09:00:09PM +0200, Laurent Vivier wrote:
>>>> qemu_set_nonblock() checks that the file descriptor can be used and, if
>>>> not, crashes QEMU. An assert() is used for that. The use of assert() is
>>>> used to detect programming error and the coredump will allow to debug
>>>> the problem.
>>>>
>>>> But in the case of the tap device, this assert() can be triggered by
>>>> a misconfiguration by the user. At startup, it's not a real problem,
>>>> but it
>>>> can also happen during the hot-plug of a new device, and here it's a
>>>> problem because we can crash a perfectly healthy system.
>>> If the user/mgmt app is not correctly passing FDs, then there's a whole
>>> pile of bad stuff that can happen. Checking whether the FD is valid is
>>> only going to catch a small subset. eg consider if fd=9 refers to the
>>> FD that is associated with the root disk QEMU has open. We'll fail to
>>> setup the TAP device and close this FD, breaking the healthy system
>>> again.
>>>
>>> I'm not saying we can't check if the FD is valid, but lets be clear that
>>> this is not offering very much protection against a broken mgmt apps
>>> passing bad FDs.
>>>
>> I agree with you, but my only goal here is to avoid the crash in this
>> particular case.
>>
>> The punishment should fit the crime.
>>
>> The user can think the netdev_del doesn't close the fd, and he can try
>> to reuse it. Sending back an error is better than crashing his system.
>> After that, if the system crashes, it will be for the good reasons, not
>> because of an assert.
> 
> 
> Yes. And on top of this we may try to validate the TAP via st_dev
> through fstat[1].

I agree, but the problem I have is to know which major(st_dev) we can
allow to use.

Do we allow only macvtap major number?
How to know the macvtap major number at user level?
[it is allocated dynamically: do we need to parse /proc/devices?]

Thanks,
Laurent