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

[Kees Cook]

On Mon, Sep 8, 2014 at 3:39 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Fri, Sep 05, 2014 at 08:41:08PM +0100, Kees Cook wrote:
>> On Thu, Sep 4, 2014 at 10:27 AM, Will Deacon <will.deacon@xxxxxxx> 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@xxxxxxx> 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.
>
> But was that hardware actually affected by the erratum? There is a runtime
> check which will avoid the cross-call if not.
>
>> Do you have a suggestion about what needs fixing?
>
> The easiest thing to do would be ensuring that __set_fixmap is always called
> with interrupts enabled. If you can guarantee that, then you need to rework
> the locking so that interrupts aren't disabled. I admit that I've lost a
> fair amount of the context here, but that's the fundamental issue.

In retesting, it hit the problem. Ugh. I will investigate what can be done.

Thanks,

-Kees

-- 
Kees Cook
Chrome OS Security
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