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

[Dave Hansen]

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?