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

Kees Cook
2014-09-11T16:27:06Z

On Thu, Sep 11, 2014 at 9:16 AM, Will Deacon <will.deacon@arm.com> wrote:
> On Thu, Sep 11, 2014 at 05:05:14PM +0100, Kees Cook wrote:
>> Actually, this doesn't make sense. If we're using
>> local_flush_tlb_kernel_range() in set_fixmap, we must always run under
>> stop_machine. The needs-broadcast case is solved by using
>> local_flush_tlb_kernel_range(), and the TLB-miss-on-other-CPU case is
>> solved by using stop_machine(). This is how the ftrace case work,
>> though not via fixmap.
>>
>> Since we need to flush the TLB on each fixmap change during
>> patch_text(), if we want to make the local_flush_tlb_... optionally
>> use flush_tlb_... to avoid calling stop_machine in the
>> does't-need-broadcast case, then we'd be checking in multiple places,
>> making this code overly complex for this rare operation. Is there a
>> good reason to complicate this code to avoid stop_machine()?
>>
>> I think we should just do this:
>>
>> diff --git a/arch/arm/kernel/patch.c b/arch/arm/kernel/patch.c
>> index 07314af47733..5038960e3c55 100644
>> --- a/arch/arm/kernel/patch.c
>> +++ b/arch/arm/kernel/patch.c
>> @@ -60,16 +125,5 @@ void __kprobes patch_text(void *addr, unsigned int insn)
>> .insn = insn,
>> };
>>
>> - if (cache_ops_need_broadcast()) {
>> - stop_machine(patch_text_stop_machine, &patch, cpu_online_mask);
>> - } else {
>> - bool straddles_word = IS_ENABLED(CONFIG_THUMB2_KERNEL)
>> - && __opcode_is_thumb32(insn)
>> - && ((uintptr_t)addr & 2);
>> -
>> - if (straddles_word)
>> - stop_machine(patch_text_stop_machine, &patch, NULL);
>> - else
>> - __patch_text(addr, insn);
>> - }
>> + stop_machine(patch_text_stop_machine, &patch, NULL);
>
> The reason not to use stop machine is that it's very expensive and the
> architecture does provide some guarantees that mean it's not always
> required...
>
> ... however, as I pointed out in a kprobes thread the other day, the code
> you remove above is actually broken, so I wouldn't be against replacing it
> all with stop_machine.

Okay, sounds good.

> Is there any way to you can WARN_ON(!called_under_stop_machine) in
> set_fixmap before doing the local TLBI?

I'm not sure what the right tests for that would be. As I understand
it, this can be called during early boot too, but I'm not sure how to
distinguish either being early or being under stop machine. Rabin or
Rob, I think you've worked on this from the perspective of early boot,
any ideas?

In the meantime, I could add a comment on set_fixmap...

>
> Will
>
> P.S. If you plan on doing an equivalent series for arm64, you should do it
> before we have any broken CPUs, then you can use TLBI without worry ;)

Heh. Hopefully I can start testing arm64 soon. I think Laura sent a
series for CONFIG_DEBUG_RODATA already, so I'm hoping by the time I
get 64-bit hardware, this will already be done. ;)

-Kees

--
Kees Cook
Chrome OS Security