Re: [PATCH v2 3/5] x86/mm: check exec permissions on fault

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 21, 2021 at 05:21:10AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>

> Add a check to prevent access_error() from returning mistakenly that
> page-faults due to instruction fetch are not allowed. Intel SDM does not
> indicate whether "instruction fetch" and "write" in the hardware error
> code are mutual exclusive, so check both before returning whether the
> access is allowed.

Dave, can we get that clarified? It seems a bit naf and leads to
confusing code IMO.

Other than that, the change looks ok to me.

> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index b2eefdefc108..e776130473ce 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1100,10 +1100,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
>  				       (error_code & X86_PF_INSTR), foreign))
>  		return 1;
>  
> -	if (error_code & X86_PF_WRITE) {
> +	if (error_code & (X86_PF_WRITE | X86_PF_INSTR)) {
>  		/* write, present and write, not present: */
> -		if (unlikely(!(vma->vm_flags & VM_WRITE)))
> +		if ((error_code & X86_PF_WRITE) &&
> +		    unlikely(!(vma->vm_flags & VM_WRITE)))
>  			return 1;
> +
> +		/* exec, present and exec, not present: */
> +		if ((error_code & X86_PF_INSTR) &&
> +		    unlikely(!(vma->vm_flags & VM_EXEC)))
> +			return 1;
> +
>  		return 0;
>  	}