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

[Beata Michalska <beata.michalska@arm.com>]

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?

> > > 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

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