Re: [PATCH RFC v2 5/6] x86: Use global pages when PTI is disabled

Dave Hansen
2018-02-15T16:55:02Z

On 02/15/2018 08:36 AM, Nadav Amit wrote:
> As long as PTI is disabled, it is possible to use global pages, as long
> as we remove them once PTI is enabled again. To do so, return the global
> bit to __supported_pte_mask and disable global pages using CR4.
>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
> arch/x86/include/asm/tlbflush.h | 6 ++++++
> arch/x86/mm/init.c | 14 ++++++--------
> arch/x86/mm/tlb.c | 3 ++-
> 3 files changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index ea65cf951c49..3a44cb0a9f56 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -319,6 +319,12 @@ static inline void set_cpu_pti_disable(unsigned short disable)
> WARN_ON_ONCE(preemptible());
>
> pti_update_user_cs64(cpu_pti_disable(), disable);
> + if (__supported_pte_mask & _PAGE_GLOBAL) {
> + if (disable)
> + cr4_set_bits(X86_CR4_PGE);
> + else
> + cr4_clear_bits(X86_CR4_PGE);
> + }
> this_cpu_write(cpu_tlbstate.pti_disable, disable);
> }

The TLB invalidations when doing this switch are *CRITICAL*. Otherwise,
we end up globally-mapped kernel entries persisting to other processes
that are then vulnerable to Meltdown.

So, where are the TLB flushes?

They're hidden in the cr4_set/clear_bits() function, of course. This is
dangerous for two reasons because it makes them non-obvious and hard to
find. It also has no interactions with the existing TLB invalidation
infrastructure. That's _safe_ of course because extra flushing is OK,
but it feels really funky because you're going to end up double-flushing
on context switches which is rather unfortunate.

This also needs some heavy commenting about the fact that _PAGE_GLOBAL
is ignored when CR4.PGE=0. That's key to this working and not mentioned
anywhere.

While this looks OK to me, it still makes me rather nervous. The
changelog and commenting definitely need a lot of work. I'm also still
rather unconvinced that the added complexity here is worth it.

> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index c67ef3fb4f35..979c7ec6baab 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -74,7 +74,8 @@ static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
> return;
> }
>
> - if (this_cpu_read(cpu_tlbstate.invalidate_other))
> + if (this_cpu_read(cpu_tlbstate.invalidate_other) &&
> + !mm_pti_disable(next))
> clear_asid_other();

This isn't obviously correct. Don't we still need to invalidate other
user asids?