linux-kernel - Re: [PATCH] x86/cpuid: Fix up "virtual" IBRS/IBPB/STIBP feature bits on Intel

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: <1517311693.18619.102.camel@infradead.org>
Date:   Tue, 30 Jan 2018 11:28:13 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     arjan@...ux.intel.com, tglx@...utronix.de, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org
Subject: Re: [PATCH] x86/cpuid: Fix up "virtual" IBRS/IBPB/STIBP feature
 bits on Intel

On Tue, 2018-01-30 at 12:18 +0100, Borislav Petkov wrote:
> On Tue, Jan 30, 2018 at 11:03:50AM +0000, David Woodhouse wrote:
> > 
> > I pondered that, but I didn't like it. I didn't want to always *force*
> > those features on, for all CPUs, just because they happened to be
> > discovered at boot time on the first CPU (which *did* have its
> > microcode updated by the crappy BIOS, while the others didn't).
> > 
> > I strongly suspect that's purely an academic concern, and we mostly
> > check boot_cpu_has() and never even *notice* if secondary CPUs don't
> > match. I just didn't want to make that *worse*. It tickled my OCD.
>
> Well, you need to do it because those bits are AMD-specific and they are
> not set in the Intel CPUID leaf and identify_cpu() towards the end takes
> care of "ironing" all those bits out which are not part of the common
> feature set and which get_cpu_cap() has *not* read out from CPUID.

I need to set them for each CPU which has the Intel hardware bits set,
sure. I don't need to use setup_force_cpu_cap() to do it. The patch I
sent was doing it for each CPU.

> It is one of those I-told-you-so moments when I suggested to make the
> visible feature bits the artificial ones and have the *actual* hardware
> ones set those.

We don't have artificial ones for the hardware capability, but yes I
could add another three. I could add X86_FEATURE_IBRS which is a
virtual bit, set when *either* X86_FEATURE_SPEC_CTRL (on Intel) or
X86_FEATURE_AMD_IBRS (on AMD) is set.

But actually... that doesn't help, does it? Because early_init_intel()
is still only called *once* for the boot CPU. Those software bits would
be set... and perhaps not later cleared when identify_boot_cpu()
happens later, but would they ever get set for secondary CPUs? The code
to set those virtual bits would *still* need to live somewhere that
will get called for secondary CPUs, as I've done in this patch.

I could use setup_force_cpu_cap() but I still don't like that, as
discussed.

So no, I don't see why inventing three more "virtual" bits to precisely
parallel the AMD bits would really make much difference.
Download attachment "smime.p7s" of type "application/x-pkcs7-signature" (5213 bytes)