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

[Nadav Amit]

> On Jul 10, 2019, at 7:22 AM, Jiri Kosina <jikos@xxxxxxxxxx> wrote:
> 
> 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.

Even with DEFINE_STATIC_KEY_FALSE_RO(), I presume you can just call
set_memory_rw(), make the page that holds the key writable, and then call
static_key_disable(), followed by a call to native_write_cr4().