[PATCH] x86/tsc: Prevent NULL pointer deref in calibrate_delay_is_known()

[Thomas Gleixner]

Subject: x86/tsc: Prevent NULL pointer deref in calibrate_delay_is_known()
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Fri, 18 Mar 2016 08:35:29 +0100

The topology_core_cpumask is used to find a neighbour cpu in
calibrate_delay_is_known(). It might not be allocated at the first invocation
of that function on the boot cpu, when CONFIG_CPUMASK_OFFSTACK is set.

The mask is allocated later in native_smp_prepare_cpus. As a consequence the
underlying find_next_bit() call dereferences a NULL pointer.

Add a proper check to prevent this.

Reported-by: Richard W.M. Jones <rjones@xxxxxxxxxx>
Fixes: c25323c07345 "x86/tsc: Use topology functions"
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx>
---
 arch/x86/kernel/tsc.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1306,11 +1306,15 @@ void __init tsc_init(void)
 unsigned long calibrate_delay_is_known(void)
 {
 	int sibling, cpu = smp_processor_id();
+	struct cpumask *mask = topology_core_cpumask(cpu);
 
 	if (!tsc_disabled && !cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC))
 		return 0;
 
-	sibling = cpumask_any_but(topology_core_cpumask(cpu), cpu);
+	if (!mask)
+		return 0;
+
+	sibling = cpumask_any_but(mask, cpu);
 	if (sibling < nr_cpu_ids)
 		return cpu_data(sibling).loops_per_jiffy;
 	return 0;