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

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi Beata,

On Thursday 25 Apr 2024 at 16:27:37 (+0200), Beata Michalska wrote:
> On Wed, Apr 24, 2024 at 11:25:27AM +0100, Sudeep Holla wrote:
> > 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@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.
> >
> We could actually move it up further to amu_fie_setup and potentially save on
> memory if none of the present CPUs have valid AMU counters. This is unlikely but
> still. So it could look like:
> 
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@@ -297,7 -194,7 +297,8 @@@ static void amu_fie_setup(const struct
>         int cpu;
> 
>         /* We are already set since the last insmod of cpufreq driver */
> ++      if (cpumask_available(amu_fie_cpus) &&
> --      if (unlikely(cpumask_subset(cpus, amu_fie_cpus)))
> ++          unlikely(cpumask_subset(cpus, amu_fie_cpus)))
>                 return;
> 
>         for_each_cpu(cpu, cpus) {
> @@@ -305,6 -202,6 +306,10 @@@
>                         return;
>         }
> 
> ++      if (!cpumask_available(amu_fie_cpus) &&
> ++              !zalloc_cpumask_var(&amu_fie_cpus, GFP_KERNEL))
> ++              return;
> ++
> 
> In both cases we risk not setting up AMUs for FIE for all or some CPUs
> if we fail to allocate the memory but I guess we are already there.
> @Ionela: What do you think?

It looks good to me. Many thanks for the fix.

Ionela.

> 
> > > > 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 ?
> Yes, the #if is needed to exclude it from compilation if !CPUMASK_OFFSTACK.
> 
> ---
> BR
> Beata
> > 
> > --
> > Regards,
> > Sudeep