Re: [GIT pull] x86/pti for 5.4-rc1

[Song Liu <songliubraving@fb.com>]

> On Sep 17, 2019, at 4:35 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> On Tue, Sep 17, 2019 at 4:29 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> How about we just do:
>> 
>> diff --git i/arch/x86/mm/pti.c w/arch/x86/mm/pti.c
>> index b196524759ec..0437f65250db 100644
>> --- i/arch/x86/mm/pti.c
>> +++ w/arch/x86/mm/pti.c
>> @@ -341,6 +341,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
>>                }
>> 
>>                if (pmd_large(*pmd) || level == PTI_CLONE_PMD) {
>> +                       WARN_ON_ONCE(addr & ~PMD_MASK);
>>                        target_pmd = pti_user_pagetable_walk_pmd(addr);
>>                        if (WARN_ON(!target_pmd))
>>                                return;
>> 
>> So it is a "warn and continue" check just for unaligned PMD address.
> 
> The problem there is that the "continue" part can be wrong.
> 
> Admittedly it requires a pretty crazy setup: you first hit a
> pmd_large() entry, but the *next* pmd is regular, so you start doing
> the per-page cloning.
> 
> And that per-page cloning will be wrong, because it will start in the
> middle of the next pmd, because addr wasn't aligned, and the previous
> pmd-only clone did
> 
>                        addr += PMD_SIZE;
> 
> to go to the next case.
> 
> See?

I see. This is tricky. 

Maybe we should skip clone of the first unaligned large pmd?

diff --git i/arch/x86/mm/pti.c w/arch/x86/mm/pti.c
index 7f2140414440..1dfa69f8196b 100644
--- i/arch/x86/mm/pti.c
+++ w/arch/x86/mm/pti.c
@@ -343,6 +343,11 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
                }

                if (pmd_large(*pmd) || level == PTI_CLONE_PMD) {
+                       if (WARN_ON_ONCE(addr & ~PMD_MASK)) {
+                               addr = round_up(addr, PMD_SIZE);
+                               continue;
+                       }
+
                        target_pmd = pti_user_pagetable_walk_pmd(addr);
                        if (WARN_ON(!target_pmd))
                                return;

Or we can round_down the addr and copy the whole PMD properly:

diff --git i/arch/x86/mm/pti.c w/arch/x86/mm/pti.c
index 7f2140414440..bee9881f2e85 100644
--- i/arch/x86/mm/pti.c
+++ w/arch/x86/mm/pti.c
@@ -343,6 +343,9 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
                }

                if (pmd_large(*pmd) || level == PTI_CLONE_PMD) {
+                       if (WARN_ON_ONCE(addr & ~PMD_MASK))
+                               addr &= PMD_MASK;
+
                        target_pmd = pti_user_pagetable_walk_pmd(addr);
                        if (WARN_ON(!target_pmd))
                                return;

I think the latter is better, but I am not sure. 

Thanks,
Song