linux-kernel - Re: [PATCH] x86: ignore spurious faults

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Date:	Wed, 23 Jan 2008 16:26:31 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Harvey Harrison <harvey.harrison@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [PATCH] x86: ignore spurious faults

Harvey Harrison wrote:
> 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;
>   

Oops, thanks.  Overlooked that one.

> 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); }
>   

Thanks for reminding me; I thought of this the other day, but forgot to 
do it.  Will post an updated patch shortly.

    J
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