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

[Andrew Cooper]

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@xxxxxxxxx> 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.

~Andrew