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

[Christian Borntraeger <borntraeger@de.ibm.com>]

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 :-)