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

[Dave Hansen]

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@xxxxxxxxxx>
> ---
>  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?