Re: [BUG] safe_smp_process_id() uses apicid which exceeds NR_CPUs in array

[Andi Kleen <ak@suse.de>]

> 
> I noticed the:   if (x86_cpu_to_apicid[apicid] == apicid)
> above.

You're right - the fast check should either check for >= NR_CPUS 
or just be removed and let it be done by the loop. I came up
with this patch.

Thanks.

-Andi

Fix fast check in safe_smp_processor_id

The APIC ID returned by hard_smp_processor_id can be beyond
NR_CPUS and then overflow the x86_cpu_to_apic[] array.

Add a check for overflow. If it happens then the slow loop below
will catch.

Bug pointed out by Doug Thompson
Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/arch/x86_64/kernel/smp.c
===================================================================
--- linux.orig/arch/x86_64/kernel/smp.c
+++ linux/arch/x86_64/kernel/smp.c
@@ -520,13 +520,13 @@ asmlinkage void smp_call_function_interr
 
 int safe_smp_processor_id(void)
 {
-	int apicid, i;
+	unsigned apicid, i;
 
 	if (disable_apic)
 		return 0;
 
 	apicid = hard_smp_processor_id();
-	if (x86_cpu_to_apicid[apicid] == apicid)
+	if (apicid < NR_CPUS && x86_cpu_to_apicid[apicid] == apicid)
 		return apicid;
 
 	for (i = 0; i < NR_CPUS; ++i) {