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

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Date:   Thu, 25 Aug 2022 10:38:23 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org, keescook@...omium.org
Subject: Re: [PATCH] x86/mm: Refuse W^X violations

On 8/25/22 10:18, Sean Christopherson wrote:
>> +/*
>> + * 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.

Good catch.  Nit: I'd probably write this up as:
	
	if (!cpu_feature_enabled(X86_FEATURE_NX))
		return new;

That gets us our fancy static branches and is a bit easier on the eyes.
 I checked and don't see a way for __supported_pte_mask to have _PAGE_NX
clear when X86_FEATURE_NX==1.