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SubjectRe: [GIT PULL] x86/topology changes for v5.3
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> On Jul 10, 2019, at 7:22 AM, Jiri Kosina <jikos@kernel.org> 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().
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