linux-kernel - Re: [kernel-hardening] [PATCH v5 03/32] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated

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Message-ID: <20160722201104.GA6613@gmail.com>
Date:	Fri, 22 Jul 2016 22:11:05 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [kernel-hardening] [PATCH v5 03/32] x86/cpa: In populate_pgd,
 don't set the pgd entry until it's populated


* Andy Lutomirski <luto@...capital.net> wrote:

> This garbage (as evidenced by my bug and my failed attempt to fix it)
> only works if you never have a low-level page table that isn't linked
> into a higher-level page table, and it mostly requires you to do
> everything exactly the way it was originally done so all the horrible
> inline helpers don't get confused.
> 
> And AFAICT all of this was done to manually unroll a loop, and I bet
> it never sped anything up measurably even on 386 or PPro.
> 
> Whenever some vendor releases a 5 level page table CPU, can we
> *please* clean this up first?  We should have a type that points to a
> table, a different type that points to an entry (or maybe not have
> pointers to entries at all), and the levels should be referred to by
> *number*.  When you need to traverse all the way down, you write a
> *loop* instead of four bloody helper functions, some of which are
> incomprehensibly no-ops on some kernels.  And if this means that, on
> Intel, we have a silly branch in the inner loop because the bottom
> level entry format is special, who cares?
> </rant>

I'd welcome (and help out!) any effort to clean it up gradually.

Thanks,

	Ingo