Re: [PATCH] x86: ignore spurious faults

[Jeremy Fitzhardinge]

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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