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

Kees Cook
2014-09-11T15:27:10Z

On Wed, Sep 10, 2014 at 10:51 AM, Will Deacon <will.deacon@arm.com> wrote:
> Hi Kees,
>
> On Tue, Sep 09, 2014 at 03:33:11PM +0100, Kees Cook wrote:
>> On Tue, Sep 9, 2014 at 5:38 AM, Will Deacon <will.deacon@arm.com> wrote:
>> > On Mon, Sep 08, 2014 at 11:40:43PM +0100, Kees Cook wrote:
>> >> Ah, so it was, yes! Will, which version of this logic would you prefer?
>> >
>> > I still don't think we're solving the general problem here -- we're actually
>> > just making the ftrace case work. It is perfectly possible for another CPU
>> > to undergo a TLB miss and refill whilst the page table is being modified by
>> > the CPU with preemption disabled. In this case, a local tlb flush won't
>> > invalidate that entry on the other core, and we have no way of knowing when
>> > the original permissions are actually observed across the system.
>>
>> The fixmap is used by anything doing patching _except_ ftrace,
>> actually. It's used by jump labels, kprobes, and kgdb. This code is
>> the general case. Access to set_fixmap is done via the kernel patching
>> interface: patch_text().
>>
>> Right now, the patch_text interface checks cache_ops_need_broadcast(),
>> and conditionally runs under stop_machine(). We could make this
>> unconditional, and we'll avoid any problem with TLB misses on another
>> CPU.
>
> Yes, it we always use stop_machine, that solves the TLB broadcast problem
> and we could do that if CONFIG_ARM_ERRATA_798181 is set.

Okay, sounds good.

>
>> > So I think we need to figure out a way to invalidate the TLB properly. What
>> > do architectures that use IPIs for TLB broadcasting do (x86, some powerpc,
>> > mips, ...)? They must have exactly the same problem.
>>
>> I don't think this should be done at the set_fixmap level, as it is
>> more a primitive. I think making sure patch_text() always works would
>> be best. What do you think of using an unconditional stop_machine()
>> instead?
>
> Why not move the TLB invalidation into patch_text, then we can do
> stop_machine if IS_ENABLED(CONFIG_ARM_ERRATA_798181) ||
> tlb_ops_need_broadcast()?

The (local) TLB flush needs to happen for patch_text to do its work,
so I'd rather it stay in set_fixmap, otherwise the flush calls will
have to follow each call of set_fixmap.

Is there a reason tlb_ops_need_broadcast() doesn't check
IS_ENABLED(CONFIG_ARM_ERRATA_798181) itself?

> Then that just leaves ftrace.

ftrace is already covered by stop_machine. Is there something I missing there?

-Kees

--
Kees Cook
Chrome OS Security