Messages in this thread | | | Date | Thu, 25 Apr 2024 16:27:37 +0200 | From | Beata Michalska <> | Subject | Re: [PATCH v5 2/5] arm64: amu: Rule out potential use after free |
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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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