Re: [PATCH] x86: ignore spurious faults

Jeremy Fitzhardinge
2008-01-24T00:26:41Z

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