Re: [PATCH] x86: ignore spurious faults

Harvey Harrison
2008-01-24T00:18:31Z

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