[Devel] [PATCH vz8 v2 1/2] x86, cpuinfo: Fix race on parallel /proc/cpuinfo read

[Kirill Tkhai]

On 02.11.2020 20:13, Andrey Ryabinin wrote:
> If several threads read /proc/cpuinfo some can see in 'flags'
> values from c->x86_capability, before __do_cpuid_fault() called
> and masks applied. Fix this by forming 'flags' on stack first
> and copy them in per_cpu(cpu_flags, cpu) as a last step.
> 
> https://jira.sw.ru/browse/PSBM-121823
> Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
> ---
> Changes since v1:
>  - none
>  
>  arch/x86/kernel/cpu/proc.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
> index 4fe1577d5e6f..4cc2951e34fb 100644
> --- a/arch/x86/kernel/cpu/proc.c
> +++ b/arch/x86/kernel/cpu/proc.c
> @@ -69,11 +69,11 @@ static DEFINE_PER_CPU(struct cpu_flags, cpu_flags);
>  static void init_cpu_flags(void *dummy)
>  {
>  	int cpu = smp_processor_id();
> -	struct cpu_flags *flags = &per_cpu(cpu_flags, cpu);
> +	struct cpu_flags flags;
>  	struct cpuinfo_x86 *c = &cpu_data(cpu);
>  	unsigned int eax, ebx, ecx, edx;
>  
> -	memcpy(flags->val, c->x86_capability, NCAPINTS * sizeof(u32));
> +	memcpy(&flags, c->x86_capability, sizeof(flags));
>  
>  	/*
>  	 * Clear feature bits masked using cpuid masking/faulting.
> @@ -81,26 +81,27 @@ static void init_cpu_flags(void *dummy)
>  
>  	if (c->cpuid_level >= 0x00000001) {
>  		__do_cpuid_fault(0x00000001, 0, &eax, &ebx, &ecx, &edx);
> -		flags->val[4] &= ecx;
> -		flags->val[0] &= edx;
> +		flags.val[4] &= ecx;
> +		flags.val[0] &= edx;
>  	}
>  
>  	if (c->cpuid_level >= 0x00000007) {
>  		__do_cpuid_fault(0x00000007, 0, &eax, &ebx, &ecx, &edx);
> -		flags->val[9] &= ebx;
> +		flags.val[9] &= ebx;
>  	}
>  
>  	if ((c->extended_cpuid_level & 0xffff0000) == 0x80000000 &&
>  	    c->extended_cpuid_level >= 0x80000001) {
>  		__do_cpuid_fault(0x80000001, 0, &eax, &ebx, &ecx, &edx);
> -		flags->val[6] &= ecx;
> -		flags->val[1] &= edx;
> +		flags.val[6] &= ecx;
> +		flags.val[1] &= edx;
>  	}
>  
>  	if (c->cpuid_level >= 0x0000000d) {
>  		__do_cpuid_fault(0x0000000d, 1, &eax, &ebx, &ecx, &edx);
> -		flags->val[10] &= eax;
> +		flags.val[10] &= eax;
>  	}
> +	memcpy(&per_cpu(cpu_flags, cpu), &flags, sizeof(flags));

This is still racy, since memcpy() is not atomic. Maybe we should add some lock on top of this?

>  }
>  
>  static int show_cpuinfo(struct seq_file *m, void *v)
>