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Subject: Re: [PATCH] x86: ignore spurious faults
From: Harvey Harrison <harvey.harrison@gmail.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Ingo Molnar <mingo@elte.hu>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andi Kleen <ak@suse.de>
In-Reply-To: <4797D64D.1060105@goop.org>
References: <4797D64D.1060105@goop.org>
Content-Type: text/plain
Date: Wed, 23 Jan 2008 16:18:36 -0800
Message-Id: <1201133916.16972.124.camel@brick>
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On Wed, 2008-01-23 at 16:05 -0800, Jeremy Fitzhardinge wrote:
> ===================================================================
> --- a/arch/x86/mm/fault_32.c
> +++ b/arch/x86/mm/fault_32.c
> @@ -290,6 +290,53 @@ static int is_errata93(struct pt_regs *r
>  
> 
>  /*
> + * Handle a spurious fault caused by a stale TLB entry.  This allows
> + * us to lazily refresh the TLB when increasing the permissions of a
> + * kernel page (RO -> RW or NX -> X).  Doing it eagerly is very
> + * expensive since that implies doing a full cross-processor TLB
> + * flush, even if no stale TLB entries exist on other processors.
> + * There are no security implications to leaving a stale TLB when
> + * increasing the permissions on a page.
> + */
> +static int spurious_fault(unsigned long address,
> +			  unsigned long error_code)
> +{
> +	pgd_t *pgd;
> +	pud_t *pud;
> +	pmd_t *pmd;
> +	pte_t *pte;
> +
> +	/* Reserved-bit violation or user access to kernel space? */
> +	if (error_code & (PF_USER | PF_RSVD))
> +		return 0;
> +
> +	pgd = init_mm.pgd + pgd_index(address);
> +	if (!pgd_present(*pgd))
> +		return 0;
> +
> +	pud = pud_offset(pgd, address);
> +	if (!pud_present(*pud))
> +		return 0;
> +
> +	pmd = pmd_offset(pud, address);
> +	if (!pmd_present(*pmd))
> +		return 0;
> +
> +	pte = pte_offset_kernel(pmd, address);
> +	if (!pte_present(*pte))
> +		return 0;
> +	if ((error_code & 0x02) && !pte_write(*pte))
> +		return 0;

	if ((error_code & PF_WRITE) && !pte_write(*pte))
		return 0;

> +
> +#if _PAGE_NX
> +	if ((error_code & PF_INSTR) && !pte_exec(*pte))
> +		return 0;
> +#endif
> +

How about dropping the #if and rely on the !pte_exec() test always
being false when _PAGE_NX = 0?  The compiler should just trim this
all away.

from pgtable.h:

static inline int pte_exec(pte_t pte)           { return !(pte_val(pte)
& _PAGE_NX); }

Cheers,

Harvey

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