Re: [PATCH v5 3/8] arm: fixmap: implement __set_fixmap()

Kees Cook
2014-09-04T17:23:44Z

On Thu, Sep 4, 2014 at 10:03 AM, Will Deacon <will.deacon@arm.com> wrote:
> Hi Kees,
>
> On Wed, Sep 03, 2014 at 10:57:04PM +0100, Kees Cook wrote:
>> This is used from set_fixmap() and clear_fixmap() via asm-generic/fixmap.h.
>> Also makes sure that the fixmap allocation fits into the expected range.
>>
>> Based on patch by Rabin Vincent.
>
> [...]
>
>> +void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
>> +{
>> + unsigned long vaddr = __fix_to_virt(idx);
>> + pte_t *pte = pte_offset_kernel(pmd_off_k(vaddr), vaddr);
>> +
>> + /* Make sure fixmap region does not exceed available allocation. */
>> + BUILD_BUG_ON(FIXADDR_START + (__end_of_fixed_addresses * PAGE_SIZE) >
>> + FIXADDR_END);
>> + BUG_ON(idx >= __end_of_fixed_addresses);
>> +
>> + if (pgprot_val(prot))
>> + set_pte_at(NULL, vaddr, pte,
>> + pfn_pte(phys >> PAGE_SHIFT, prot));
>> + else
>> + pte_clear(NULL, vaddr, pte);
>> +
>> + /*
>> + * Given the potential a15 tlbi errata, we can only do tlb flushes
>> + * with interrupts disabled. Callers must have taken care of this.
>> + */
>> + WARN_ON_ONCE(!irqs_disabled());
>> + flush_tlb_kernel_range(vaddr, vaddr + PAGE_SIZE);
>
> Aha, this explains why we were confusing each other! The issue is that
> interrupts must be *enabled*, so this code does the exact opposite of
> what we need.
>
> I think this got lost in a sea of double negatives during the last round
> of review.

Ah! If this is the case, perhaps we can get away with
local_flush_tlb_kernel_range() then?

-Kees

--
Kees Cook
Chrome OS Security