Re: [PATCH] KVM: Forbid /dev/kvm being opened by a compat task when CONFIG_KVM_COMPAT=n

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/11/19 09:20, Christian Borntraeger wrote:
> 
> 
> On 14.11.19 09:15, Marc Zyngier wrote:
>> On Wed, 13 Nov 2019 21:23:07 +0000
>> Peter Maydell <peter.maydell@linaro.org> wrote:
>>
>>> On Wed, 13 Nov 2019 at 18:44, Christian Borntraeger
>>> <borntraeger@de.ibm.com> wrote:
>>>> On 13.11.19 17:05, Marc Zyngier wrote:  
>>>>> On a system without KVM_COMPAT, we prevent IOCTLs from being issued
>>>>> by a compat task. Although this prevents most silly things from
>>>>> happening, it can still confuse a 32bit userspace that is able
>>>>> to open the kvm device (the qemu test suite seems to be pretty
>>>>> mad with this behaviour).
>>>>>
>>>>> Take a more radical approach and return a -ENODEV to the compat
>>>>> task.  
>>>
>>>> Do we still need compat_ioctl if open never succeeds?  
>>>
>>> I wondered about that, but presumably you could use
>>> fd-passing, or just inheriting open fds across exec(),
>>> to open the fd in a 64-bit process and then hand it off
>>> to a 32-bit process to call the ioctl with. (That's
>>> probably only something you'd do if you were
>>> deliberately playing silly games, of course, but
>>> preventing silly games is useful as it makes it
>>> easier to reason about kernel behaviour.)
>>
>> This was exactly my train of thoughts, which I should have made clear
>> in the commit log. Thanks Peter for reading my mind! ;-)
> 
> Makes sense. Looks like this is already in kvm/master so we cannot improve
> the commit message easily any more. Hopefully we will not forget :-)

A comment in the code would probably be better than the commit message,
to not forget stuff like this.  (Hint! :))

Paolo