Re: [PATCH] x86/mm: Refuse W^X violations

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 25, 2022, Peter Zijlstra wrote:
> 
> x86 has STRICT_*_RWX, but not even a warning when someone violates it.
> 
> Add this warning and fully refuse the transition.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 1abd5438f126..9e9bef3f36b3 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -579,6 +579,30 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long start,
>  	return __pgprot(pgprot_val(prot) & ~forbidden);
>  }
>  
> +/*
> + * Validate and enforce strict W^X semantics.
> + */
> +static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long start,
> +				  unsigned long pfn, unsigned long npg)
> +{
> +	unsigned long end;
> +

I think this needs

	if (!(__supported_pte_mask & _PAGE_NX))
		return new;

to play nice with non-PAE 32-bit kernels.

> +	if (!((pgprot_val(old) ^ pgprot_val(new)) & (_PAGE_RW | _PAGE_NX)))
> +		return new;
> +
> +	if ((pgprot_val(new) & (_PAGE_RW | _PAGE_NX)) != _PAGE_RW)
> +		return new;
> +
> +	end = start + npg * PAGE_SIZE - 1;
> +	WARN(1, "CPA refuse W^X violation: %016llx -> %016llx range: 0x%016lx - 0x%016lx PFN %lx\n",

WARN_ONCE() to avoid eternal spam if something does go sideways?

> +	     (unsigned long long)pgprot_val(old),
> +	     (unsigned long long)pgprot_val(new),
> +	     start, end, pfn);
> +
> +	/* refuse the transition into WX */
> +	return old;
> +}