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

[Ingo Molnar <mingo@kernel.org>]

* Song Liu <songliubraving@fb.com> wrote:

> 
> 
> > 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;

No, we should do a proper iteration of the page table structures.

> 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. 

While this works, it's the wrong iterator pattern I believe.

In this function we iterate by passing in a 'random' [start,end) virtual 
memory address range with no particular alignment assumptions, then look 
up all pagetable entries covered by that range.

The iteration's principle is straightforward: we look up the first 
address (byte granular) then continue iterating according to the observed 
structure of the kernel pagetables, by skipping the range we have just 
looked up:

- If the current PUD is not mapped, then we set 'addr' to the first byte 
  after the virtual memory range represented by the current PUD entry:

    addr = round_up(addr + 1, PUD_SIZE);

- If the current PMD is not mapped, then the next byte is:

    addr = round_up(addr + 1, PMD_SIZE);

The part Linus correctly pointed it is still iterating incorrectly and 
might potentially be unrobust is:

    addr += PMD_SIZE;

This is buggy because it doesn't step to the next byte after the current 
mapped PMD, but potentially somewhere into the middle of the next 
PMD-sized range of virtual memory (which might or might not be covered by 
a PMD entry). The iterations after that might be similarly offset and 
buggy as well.

The right fix is to *fix the address iterator*, to use the basic 
principle of the function, with the same general exact calculation 
pattern we use in the other cases:

    addr = round_down(addr, PMD_SIZE) + PMD_SIZE;

BTW., I'd also suggest using this new round_down() pattern in the other 
two cases as well:

    addr = round_down(addr, PUD_SIZE) + PUD_SIZE;
    ...
    addr = round_down(addr, PMD_SIZE) + PMD_SIZE;

Why? Because this:

    addr = round_up(addr + 1, PUD_SIZE);

Will iterate incorrectly if 'addr' (which is byte granular) is the last 
*byte* of a PUD range, it will incorrectly skip the next PUD range...

Is a page-unaligned address likely to be passed in to this function? With 
the current users I really hope it won't happen, but it costs nothing to 
use clean iterators and think through all cases - it also makes the code 
more readable.

Three random nits about the pti_clone_pgtable() function:

- Could we please also fix all WARN()'s in that function to be 
  WARN_ONCE()? Any warning from that function is probably fatal to the 
  bootup anyway, and it doesn't help if we potentially spam many 
  warnings.

- Please add an explanation comment to why the 'BUG();' case is 
  unrecoverable and needs us to crash the kernel.

- Please add a comment about what the 'level' parameter does. It's non-obvious.

Thanks,

	Ingo