[PATCH v5 2/5] arm64: amu: Rule out potential use after free

[Sudeep Holla]

On Thu, Apr 18, 2024 at 05:55:43PM +0200, Beata Michalska wrote:
> On Thu, Apr 18, 2024 at 11:50:52AM +0100, Sudeep Holla wrote:
> > On Wed, Apr 17, 2024 at 10:38:45AM +0100, Beata Michalska wrote:
> > > For the time being, the amu_fie_cpus cpumask is being exclusively used
> > > by the AMU-related internals of FIE support and is guaranteed to be
> > > valid on every access currently made. Still the mask is not being
> > > invalidated on one of the error handling code paths, which leaves
> > > a soft spot with potential risk of uaf for CPUMASK_OFFSTACK cases.
> > > To make things sound, set the cpumaks pointer explicitly to NULL upon
> > > failing to register the cpufreq notifier.
> > > Note that, due to the quirks of CPUMASK_OFFSTACK, this change needs to
> > > be wrapped with grim ifdefing (it would be better served by
> > > incorporating this into free_cpumask_var ...)
> > >
> >
> > Yes it doesn't look neat.
> >
> > > Signed-off-by: Beata Michalska <beata.michalska at arm.com>
> > > ---
> > >  arch/arm64/kernel/topology.c | 6 +++++-
> > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > > index 1a2c72f3e7f8..3c814a278534 100644
> > > --- a/arch/arm64/kernel/topology.c
> > > +++ b/arch/arm64/kernel/topology.c
> > > @@ -244,8 +244,12 @@ static int __init init_amu_fie(void)
> > >
> > >  	ret = cpufreq_register_notifier(&init_amu_fie_notifier,
> > >  					CPUFREQ_POLICY_NOTIFIER);
> > > -	if (ret)
> > > +	if (ret) {
> > >  		free_cpumask_var(amu_fie_cpus);
> > > +#ifdef CONFIG_CPUMASK_OFFSTACK
> > > +		amu_fie_cpus = NULL;
> > > +#endif
> > > +	}
> >
> > Instead of this #ifdeffery, I was wondering if we can actually do the
> > allocation in init_amu_fie_callback() the first time it gets called
> > checking if amu_fie_cpus is NULL. init_amu_fie_callback() must get called
> > only if the cpufreq_register_notifier() succeeds right ?
> >

> Delayed allocation ... I guess this will do the trick.

I prefer that if we can't find any other alternative. Do you see any issues
with that ? That said I am fine if Will/Catalin is happy with this.

> > Also I don't see anyone calling amu_fie_setup(), so where do you think
> > the possible use after free could occur for amu_fie_cpus. Just thinking
> > out loud to check if I missed anything.
> >
> You haven't missed anything. Currently the uaf is purely theoretical as the code
> that relies on that mask will only be executed if we have succeeded to register
> the amu fie support: so far so good.

Yes it is better to handle it even if it is theoretical.

I assume you get some compiler error if you assign unconditionally and
if(IS_ENABLED()) also doesn't work in this case as it would still give
error ?

--
Regards,
Sudeep