Re: [GIT PULL] x86/topology changes for v5.3

[Jiri Kosina]

On Wed, 10 Jul 2019, Peter Zijlstra wrote:

> If we mark the key as RO after init, and then try and modify the key to
> link module usage sites, things might go bang as described.
> 
> Thanks!
> 
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 27d7864e7252..5bf7a8354da2 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -366,7 +366,7 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
>  	cr4_clear_bits(X86_CR4_UMIP);
>  }
>  
> -DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
> +DEFINE_STATIC_KEY_FALSE(cr_pinning);

Good catch, I guess that is going to fix it.

At the same time though, it sort of destroys the original intent of Kees' 
patch, right? The exploits will just have to call static_key_disable() 
prior to calling native_write_cr4() again, and the protection is gone.

-- 
Jiri Kosina
SUSE Labs