linux-kernel - Re: [PATCH 03/14] x86/entry/64: Fix unwind hints in register clearing code

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200312200738.GB5086@worktop.programming.kicks-ass.net>
Date:   Thu, 12 Mar 2020 21:07:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andy Lutomirski <luto@...capital.net>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Dave Jones <dsj@...com>, Jann Horn <jannh@...gle.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 03/14] x86/entry/64: Fix unwind hints in register
 clearing code

On Thu, Mar 12, 2020 at 02:57:14PM -0500, Josh Poimboeuf wrote:
> On Thu, Mar 12, 2020 at 12:29:29PM -0700, Andy Lutomirski wrote:
> > > On Mar 12, 2020, at 10:31 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > > 
> > > The PUSH_AND_CLEAR_REGS macro zeroes each register immediately after
> > > pushing it.  If an NMI or exception hits after a register is cleared,
> > > but before the UNWIND_HINT_REGS annotation, the ORC unwinder will
> > > wrongly think the previous value of the register was zero.  This can
> > > confuse the unwinding process and cause it to exit early.
> > > 
> > > Because ORC is simpler than DWARF, there are a limited number of unwind
> > > annotation states, so it's not possible to add an individual unwind hint
> > > after each push/clear combination.  Instead, the register clearing
> > > instructions need to be consolidated and moved to after the
> > > UNWIND_HINT_REGS annotation.
> > 
> > I don’t suppose you know how bad t he performance hit is on a non-PTI machine?
> 
> Hm, what does it have to do with PTI?  Should I run a syscall
> microbenchmark?

Mostly that performance with PTI on is abysmal so we don't care about a
few cycles.