Re: [RFC 03/20] mm/mprotect: do not flush on permission promotion

Andrew Cooper
2021-02-01T15:40:57Z

On 01/02/2021 05:58, Nadav Amit wrote:
>> On Jan 31, 2021, at 4:10 AM, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>
>> On 31/01/2021 01:07, Andy Lutomirski wrote:
>>> Adding Andrew Cooper, who has a distressingly extensive understanding
>>> of the x86 PTE magic.
>> Pretty sure it is all learning things the hard way...
>>
>>> On Sat, Jan 30, 2021 at 4:16 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>>>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>>>> index 632d5a677d3f..b7473d2c9a1f 100644
>>>> --- a/mm/mprotect.c
>>>> +++ b/mm/mprotect.c
>>>> @@ -139,7 +139,8 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>>>> ptent = pte_mkwrite(ptent);
>>>> }
>>>> ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
>>>> - tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
>>>> + if (pte_may_need_flush(oldpte, ptent))
>>>> + tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
>> You're choosing to avoid the flush, based on A/D bits read ahead of the
>> actual modification of the PTE.
>>
>> In this example, another thread can write into the range (sets A and D),
>> and get a suitable TLB entry which goes unflushed while the rest of the
>> kernel thinks the memory is write-protected and clean.
>>
>> The only safe way to do this is to use XCHG/etc to modify the PTE, and
>> base flush calculations on the results. Atomic operations are ordered
>> with A/D updates from pagewalks on other CPUs, even on AMD where A
>> updates are explicitly not ordered with regular memory reads, for
>> performance reasons.
> Thanks Andrew for the feedback, but I think the patch does it exactly in
> this safe manner that you describe (at least on native x86, but I see a
> similar path elsewhere as well):
>
> oldpte = ptep_modify_prot_start()
> -> __ptep_modify_prot_start()
> -> ptep_get_and_clear
> -> native_ptep_get_and_clear()
> -> xchg()
>
> Note that the xchg() will clear the PTE (i.e., making it non-present), and
> no further updates of A/D are possible until ptep_modify_prot_commit() is
> called.
>
> On non-SMP setups this is not atomic (no xchg), but since we hold the lock,
> we should be safe.
>
> I guess you are right and a pte_may_need_flush() deserves a comment to
> clarify that oldpte must be obtained by an atomic operation to ensure no A/D
> bits are lost (as you say).
>
> Yet, I do not see a correctness problem. Am I missing something?

No(ish) - I failed to spot that path.

But native_ptep_get_and_clear() is broken on !SMP builds.  It needs to
be an XCHG even in that case, to spot A/D updates from prefetch or
shared-virtual-memory DMA.

~Andrew