Messages in this thread | | | Subject | Re: [RFC 03/20] mm/mprotect: do not flush on permission promotion | From | Andrew Cooper <> | Date | Sun, 31 Jan 2021 12:10:49 +0000 |
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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.
~Andrew
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