Re: [PATCH v3 6/7] x86/alternatives: use temporary mm for text poking

[Nadav Amit]

From: Peter Zijlstra
Sent: November 5, 2018 at 1:30:41 PM GMT
> To: Nadav Amit <namit@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, H. Peter Anvin <hpa@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxx>, Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Subject: Re: [PATCH v3 6/7] x86/alternatives: use temporary mm for text poking
> 
> 
> On Fri, Nov 02, 2018 at 04:29:45PM -0700, Nadav Amit wrote:
>> +	unuse_temporary_mm(prev);
>> +
>> +	pte_unmap_unlock(ptep, ptl);
> 
> That; that does kunmap_atomic() on 32bit.
> 
> I've been thinking that the whole kmap_atomic thing on x86_32 is
> terminally broken, and with that most of x86_32 is.
> 
> kmap_atomic does the per-cpu fixmap pte fun-and-games we're here saying
> is broken. Yes, only the one CPU will (explicitly) use those fixmap PTEs
> and thus the local invalidate _should_ work. However nothing prohibits
> speculation on another CPU from using our fixmap addresses. Which can
> lead to the remote CPU populating its TLBs for our fixmap entry.
> 
> And, as we've found, there are AMD parts that #MC when there are
> mis-matched TLB entries.
> 
> So what do we do? mark x86_32 SMP broken?

pte_unmap() seems to only use kunmap_atomic() when CONFIG_HIGHPTE is set, no?

Do most distributions run with CONFIG_HIGHPTE?