Re: [PATCH 6/9] x86/mm: Introduce ptep_set_wrprotect_flush and related functions

Yu-cheng Yu
2018-06-07T20:39:18Z

On Thu, 2018-06-07 at 13:29 -0700, Dave Hansen wrote:
> On 06/07/2018 09:24 AM, Andy Lutomirski wrote:
>
> >> +static inline void ptep_set_wrprotect_flush(struct vm_area_struct *vma,
> >> + unsigned long addr, pte_t *ptep)
> >> +{
> >> + bool rw;
> >> +
> >> + rw = test_and_clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> >> + if (IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER)) {
> >> + struct mm_struct *mm = vma->vm_mm;
> >> + pte_t pte;
> >> +
> >> + if (rw && (atomic_read(&mm->mm_users) > 1))
> >> + pte = ptep_clear_flush(vma, addr, ptep);
> > Why are you clearing the pte?
>
> I found my notes on the subject. :)
>
> Here's the sequence that causes the problem. This could happen any time
> we try to take a PTE from read-write to read-only. P==Present, W=Write,
> D=Dirty:
>
> CPU0 does a write, sees PTE with P=1,W=1,D=0
> CPU0 decides to set D=1
> CPU1 comes in and sets W=0
> CPU0 does locked operation to set D=1
> CPU0 sees P=1,W=0,D=0
> CPU0 sets back P=1,W=0,D=1
> CPU0 loads P=1,W=0,D=1 into the TLB
> CPU0 attempts to continue the write, but sees W=0 in the TLB and a #PF
> is generated because of the write fault.
>
> The problem with this is that we end up with a shadowstack-PTE
> (Write=0,Dirty=1) where we didn't want one. This, unfortunately,
> imposes extra TLB flushing overhead on the R/W->R/O transitions that
> does not exist before shadowstack enabling.
>
> Yu-cheng, could you please add this to the patch description?

I will add that.