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

[Will Deacon]

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 at 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 at 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.

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.

Will