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

From: Balbir Singh <[email-masked]>
Date: Fri, 3 Feb 2017 16:28:22 +0530

On Fri, Feb 03, 2017 at 05:10:28PM +1100, Benjamin Herrenschmidt wrote:
> 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")
> ---
> 
> 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;
>  	}

Aneesh did suggest a check for !user_mode(regs), but we did multiple combinations
of potential problems and decided the current check was OK. Not yet tested at my end,
I'll do that today.

Balbir Singh