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

[Peter Zijlstra]

On Mon, Nov 05, 2018 at 06:04:42PM +0000, Nadav Amit wrote:
> 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?

Sure; but all of x86_32 relies on kmap_atomic. This was just the the one
way I ran into it again.

By our current way of thinking, kmap_atomic simply is not correct.