Subject: Re: [PATCH] powerpc/mm: Fix spurrious segfaults on radix with Autonuma

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 03 Feb 2017 12:00:51 +0530

Benjamin Herrenschmidt <[email-masked]> writes:

> When autonuma marks a PTE inaccessible it clears all the protection
> bits but leave the PTE valid.
>
> With the Radix MMU, an attempt at executing from such a PTE will
> take a fault with bit 35 of SRR1 set "SRR1_ISI_N_OR_G".
>
> It is thus incorrect to treat all such faults as errors. We should
> pass them to handle_mm_fault() for autonuma to deal with. The case
> of pages that are really not executable is handled by the existing
> test for VM_EXEC further down.
>
> That leaves us with catching the kernel attempts at executing user
> pages. We can catch that earlier, even before we do find_vma.
>
> It is never valid on powerpc for the kernel to take an exec fault
> to begin with. So fold that test with the existing test for the
> kernel faulting on kernel addresses to bail out early.
>
> Signed-off-by: Benjamin Herrenschmidt <[email-masked]>
> Fixes: 1d18ad0 ("powerpc/mm: Detect instruction fetch denied and report")
> Fixes: 0ab5171 ("powerpc/mm: Fix no execute fault handling on pre-POWER5")

Reviewed-by: Aneesh Kumar K.V <[email-masked]>

> ---
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 6fd30ac..62a50d6 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -253,8 +253,11 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>  	if (unlikely(debugger_fault_handler(regs)))
>  		goto bail;
>
> -	/* On a kernel SLB miss we can only check for a valid exception entry */
> -	if (!user_mode(regs) && (address >= TASK_SIZE)) {
> +	/*
> +	 * The kernel should never take an execute fault nor should it
> +	 * take a page fault to a kernel address.
> +	 */
> +	if (!user_mode(regs) && (is_exec || (address >= TASK_SIZE))) {
>  		rc = SIGSEGV;
>  		goto bail;
>  	}
> @@ -391,20 +394,6 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>
>  	if (is_exec) {
>  		/*
> -		 * An execution fault + no execute ?
> -		 *
> -		 * On CPUs that don't have CPU_FTR_COHERENT_ICACHE we
> -		 * deliberately create NX mappings, and use the fault to do the
> -		 * cache flush. This is usually handled in hash_page_do_lazy_icache()
> -		 * but we could end up here if that races with a concurrent PTE
> -		 * update. In that case we need to fall through here to the VMA
> -		 * check below.
> -		 */
> -		if (cpu_has_feature(CPU_FTR_COHERENT_ICACHE) &&
> -			(regs->msr & SRR1_ISI_N_OR_G))
> -			goto bad_area;
> -
> -		/*
>  		 * Allow execution from readable areas if the MMU does not
>  		 * provide separate controls over reading and executing.
>  		 *