[PATCH] arm64: amu: Delay allocating cpumask for AMU FIE support

[PATCH] arm64: amu: Delay allocating cpumask for AMU FIE support

[Beata Michalska]

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 theoretical risk of UAF for CPUMASK_OFFSTACK cases.
To make things sound, delay allocating said cpumask
(for CPUMASK_OFFSTACK) avoiding otherwise nasty sanitising case failing
to register the cpufreq policy notifications.

Signed-off-by: Beata Michalska <beata.michalska@arm.com>
Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>
Reviewed-by: Sumit Gupta <sumitg@nvidia.com>
---

Hi All,

This change has been previously sent out as a part of a series at [1], 
still though, this one is more of a dependency for those changes, and otherwise
can be safely merged separately, thus excluding it from the above mentioned
patch-set.

[1] https://lore.kernel.org/all/20250121084435.2839280-1-beata.michalska@arm.com/

BR
Beata

 arch/arm64/kernel/topology.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index 1a2c72f3e7f8..cb180684d10d 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -194,12 +194,19 @@ static void amu_fie_setup(const struct cpumask *cpus)
 	int cpu;
 
 	/* We are already set since the last insmod of cpufreq driver */
-	if (unlikely(cpumask_subset(cpus, amu_fie_cpus)))
+	if (cpumask_available(amu_fie_cpus) &&
+	    unlikely(cpumask_subset(cpus, amu_fie_cpus)))
 		return;
 
-	for_each_cpu(cpu, cpus) {
+	for_each_cpu(cpu, cpus)
 		if (!freq_counters_valid(cpu))
 			return;
+
+	if (!cpumask_available(amu_fie_cpus) &&
+	    !zalloc_cpumask_var(&amu_fie_cpus, GFP_KERNEL)) {
+		WARN_ONCE(1, "Failed to allocate FIE cpumask for CPUs[%*pbl]\n",
+			  cpumask_pr_args(cpus));
+		return;
 	}
 
 	cpumask_or(amu_fie_cpus, amu_fie_cpus, cpus);
@@ -237,17 +244,8 @@ static struct notifier_block init_amu_fie_notifier = {
 
 static int __init init_amu_fie(void)
 {
-	int ret;
-
-	if (!zalloc_cpumask_var(&amu_fie_cpus, GFP_KERNEL))
-		return -ENOMEM;
-
-	ret = cpufreq_register_notifier(&init_amu_fie_notifier,
+	return cpufreq_register_notifier(&init_amu_fie_notifier,
 					CPUFREQ_POLICY_NOTIFIER);
-	if (ret)
-		free_cpumask_var(amu_fie_cpus);
-
-	return ret;
 }
 core_initcall(init_amu_fie);
 
-- 
2.25.1

Re: [PATCH] arm64: amu: Delay allocating cpumask for AMU FIE support

[Sudeep Holla]

On Fri, Jan 31, 2025 at 03:58:42PM +0000, 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 theoretical risk of UAF for CPUMASK_OFFSTACK cases.
> To make things sound, delay allocating said cpumask
> (for CPUMASK_OFFSTACK) avoiding otherwise nasty sanitising case failing
> to register the cpufreq policy notifications.
>

I thought I had provided my tag, sorry for that. FWIW,

Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>

-- 
Regards,
Sudeep

Re: [PATCH] arm64: amu: Delay allocating cpumask for AMU FIE support

[Will Deacon]

On Fri, 31 Jan 2025 15:58:42 +0000, 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 theoretical risk of UAF for CPUMASK_OFFSTACK cases.
> To make things sound, delay allocating said cpumask
> (for CPUMASK_OFFSTACK) avoiding otherwise nasty sanitising case failing
> to register the cpufreq policy notifications.
> 
> [...]

Applied to arm64 (for-next/fixes), thanks!

[1/1] arm64: amu: Delay allocating cpumask for AMU FIE support
      https://git.kernel.org/arm64/c/d923782b0412

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev