linux-kernel - Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

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: <20180213075540.3lkikkpgjoe6ocjk@gmail.com>
Date:   Tue, 13 Feb 2018 08:55:40 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave@...1.net>, hpa@...or.com, tglx@...utronix.de,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        dwmw@...zon.co.uk, linux-tip-commits@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Arjan van de Ven <arjan@...radead.org>
Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Feb 12, 2018 at 08:13:31AM -0800, Dave Hansen wrote:
> > On 02/12/2018 02:22 AM, Ingo Molnar wrote:
> > >> +static inline void firmware_restrict_branch_speculation_end(void)
> > >> +{
> > >> +	alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,
> > >> +			      X86_FEATURE_USE_IBRS_FW);
> > > BTW., there's a detail that only occurred to me today, this enabling/disabling 
> > > sequence is not NMI safe, and it might be called from NMI context:
> > 
> > FWIW, Tim Chen and I talked about this a bunch.  We ended up just
> > saving/restoring the MSR verbatim in the NMI handler the same way we do
> > CR3, stashing it in a high general-purpose-register (r%12?).  That costs
> > a RDMSR (at least) and an WRMSR (which you can optimize out).  We have a
> > patch for that somewhere if anybody wants it.
> 
> I would really rather not do that on the NMI path.. And if we _have_ to,
> please keep a software shadow of that MSR state, such that we can avoid
> touching that MSR 99% of the time.

Yeah, I'd rather avoid doing firmware calls from NMI context altogether.

Thanks,

	Ingo