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

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Mar 18, 2016 at 08:48:06AM +0100, Thomas Gleixner wrote:
> Subject: x86/tsc: Prevent NULL pointer deref in calibrate_delay_is_known()
> From: Thomas Gleixner <tglx@linutronix.de>
> 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@redhat.com>
> Fixes: c25323c07345 "x86/tsc: Use topology functions"
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Josh Boyer <jwboyer@fedoraproject.org>

I have tested the current upstream kernel (9dffdb38d) and was able to
reproduce the bug.  I then added this patch on top and it fixes the
problem for me.  Therefore:

Tested-by: Richard W.M. Jones <rjones@redhat.com>

Thanks, Rich.

> ---
>  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;

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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