From: Kees Cook <keescook@chromium.org>
To: Solar Designer <solar@openwall.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Jann Horn <jannh@google.com>,
Dominik Brodowski <linux@dominikbrodowski.net>,
LKML <linux-kernel@vger.kernel.org>,
Kernel Hardening <kernel-hardening@lists.openwall.com>,
X86 ML <x86@kernel.org>
Subject: Re: [PATCH v2] x86/asm: Pin sensitive CR4 bits
Date: Thu, 21 Feb 2019 08:11:08 -0800 [thread overview]
Message-ID: <CAGXu5jJ46whob0-DE1_dydRtoNAN0Lw7EUsnNvNyQzwt_g9iEg@mail.gmail.com> (raw)
In-Reply-To: <20190221130645.GA8281@openwall.com>
On Thu, Feb 21, 2019 at 5:06 AM Solar Designer <solar@openwall.com> wrote:
>
> On Wed, Feb 20, 2019 at 01:20:58PM -0800, Kees Cook wrote:
> > On Wed, Feb 20, 2019 at 10:49 AM Solar Designer <solar@openwall.com> wrote:
> > >
> > > On Wed, Feb 20, 2019 at 10:09:34AM -0800, Kees Cook wrote:
> > > > + if (WARN_ONCE((val & cr4_pin) != cr4_pin, "cr4 bypass attempt?!\n"))
> > > > + goto again;
> > >
> > > I think "goto again" is too mild a response given that it occurs after a
> > > successful write of a non-pinned value to CR4. I think it'd allow some
> > > exploits to eventually win the race: make their desired use of whatever
> > > functionality SMEP, etc. would have prevented - which may be just a few
> > > instructions they need to run - before the CR4 value is reverted after
> > > "goto again". I think it's one of those cases where a kernel panic
> > > would be more appropriate.
> >
> > It will not land upstream with a BUG() or panic(). Linus has
> > explicitly stated that none of this work can do that until it has
> > "baked" in the kernel for a couple years.
>
> OK.
>
> > In his defense, anyone sufficiently paranoid can already raise the
> > priority of a WARN() into a panic via sysctl kernel.panic_on_warn (and
> > kernel.panic_on_oops).
>
> I think there are too many uses of WARN() for anyone sane to enable
> that in production, whereas it'd have made sense to enable it for the
> few security-related uses.
Yeah, that's been my thinking too. I've been thinking about this for a
while trying to decide if we need something between WARN and BUG, but
I can't make up my mind. ;)
> > > Also, WARN_ONCE possibly introduces a delay sufficient to realistically
> > > win this race on the first try. If we choose to warn, we should do it
> > > after having reverted the CR4 value, not before.
> >
> > Isn't cr4 CPU-local though?
>
> Good point. I don't know. If CR4 is per hardware thread, then the race
> would require an interrupt and would be much harder to win.
>
> > Couldn't we turn off interrupts to stop the race?
>
> This won't help. An attack would skip the code that disables interrupts
> and land right on the MOV instruction.
Oh duh, yeah. ;)
I think v2 is good enough given the constraints we've got.
Thanks for looking at it!
--
Kees Cook
next prev parent reply other threads:[~2019-02-21 16:11 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-20 18:09 [PATCH v2] x86/asm: Pin sensitive CR4 bits Kees Cook
2019-02-20 18:48 ` Solar Designer
2019-02-20 21:20 ` Kees Cook
2019-02-21 13:06 ` Solar Designer
2019-02-21 16:11 ` Kees Cook [this message]
2019-02-21 17:33 ` Sean Christopherson
2019-02-21 17:37 ` Kees Cook
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAGXu5jJ46whob0-DE1_dydRtoNAN0Lw7EUsnNvNyQzwt_g9iEg@mail.gmail.com \
--to=keescook@chromium.org \
--cc=jannh@google.com \
--cc=kernel-hardening@lists.openwall.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux@dominikbrodowski.net \
--cc=solar@openwall.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).