Re: [RFC][PATCH 2/6] x86/power: Inline write_cr[04]()

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> Since we can't do CALL/RET until GS is restored and CR[04] pinning is
> of dubious value in this code path, simply write the stored values.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/power/cpu.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -208,11 +208,11 @@ static void notrace __restore_processor_
>  #else
>  /* CONFIG X86_64 */
>  	native_wrmsrl(MSR_EFER, ctxt->efer);
> -	native_write_cr4(ctxt->cr4);
> +	asm volatile("mov %0,%%cr4": "+r" (ctxt->cr4) : : "memory");

>  #endif
>  	native_write_cr3(ctxt->cr3);
>  	native_write_cr2(ctxt->cr2);
> -	native_write_cr0(ctxt->cr0);
> +	asm volatile("mov %0,%%cr0": "+r" (ctxt->cr0) : : "memory");

Yeah, so CR pinning protects against are easily accessible 'gadget' 
functions that exploits can call to disable HW protection features in the 
CR register.

__restore_processor_state() might be such a gadget if an exploit can pass 
in a well-prepared 'struct saved_context' on the stack.

Can we set up cr0/cr4 after we have a proper GS, or is that a 
chicken-and-egg scenario?

Thanks,

	Ingo