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

Kees Cook
2014-09-05T19:41:12Z

On Thu, Sep 4, 2014 at 10:27 AM, Will Deacon <will.deacon@arm.com> wrote:
> On Thu, Sep 04, 2014 at 06:23:42PM +0100, Kees Cook wrote:
>> 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?
>
> That's a bit tricky, since you need to ensure that preemption is disabled
> until the mapping is put back like it was.

Even with CONFIG_ARM_ERRATA_798181 enabled, I don't see a problem here
using flush_tlb_kernel_range(). When doing the ftrace work, this was
trivial to trip over, so I think we're in a good state here.

I've tested this on real hardware now too, and nothing falls over.
I'll get rid of the comment and the WARN_ON_ONCE, but AFAICT, this is
safe.

Do you have a suggestion about what needs fixing?

-Kees

--
Kees Cook
Chrome OS Security