linux-kernel - Re: [PATCH] x86/mm+efi: Avoid creating W+X mappings

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Message-ID: <202209231417.F73F40060@keescook>
Date:   Fri, 23 Sep 2022 14:19:43 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-efi@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/mm+efi: Avoid creating W+X mappings

On Fri, Sep 23, 2022 at 09:53:02PM +0200, Ard Biesheuvel wrote:
> On Fri, 23 Sept 2022 at 20:31, Kees Cook <keescook@...omium.org> wrote:
> >
> > On Fri, Sep 23, 2022 at 04:26:58PM +0200, Ard Biesheuvel wrote:
> > > I was basically making the point that we still support i386 without
> > > PAE (which is a prerequisite for supporting non-executable mappings),
> > > and if we are going to be pedantic about security on this
> > > architecture, we should probably make PAE mandatory as well.
> >
> > My expectation would be that if someone is running modern kernels on i386,
> > they're not using PAE. If they care about PAE, I'd expect them to have
> > long since moved to x86_64.
> >
> 
> Not sure I follow. If they care about PAE, they turn it on. Or do you
> mean 'if they care about being able to address lots of physical
> memory'? Because the *other* reason you might care about PAE is
> because it gives you NX support.

Right, I meant if they care about NX (and the topic of this thread)
they want PAE, and if they want PAE, they likely moved to x86_64 long
long ago for new kernels.

> But currently, PAE is not even enabled in the i386_defconfig, and
> defaults to off. This means people that are unaware of this won't
> enable it, and will be running without NX support.

And they all make me cry. ;)

> > > If we are ok with the current state, enabling this permission check on
> > > i386 makes no sense.
> >
> > I'd agree. If it's a choice between "spend a lot of time making sure
> > this works correctly on i386" and "don't do this at all on i386", I
> > would pick the latter. If someone steps up to do the former, then by
> > all means take the patches.
> >
> 
> OK, so it seems we're all in violent agreement here. And if there is
> ever a push for enabling security features on 32-bit, we can add this
> to the laundry list of things that need to be looked at.

Yup.

-- 
Kees Cook