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

[Marc Zyngier <maz@kernel.org>]

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! ;-)

	M.
-- 
Jazz is not dead. It just smells funny...