linux-kernel - Re: [PATCH v2 24/39] x86/cet/shstk: Add user-mode shadow stack support

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Message-ID: <202210032101.A33914E4@keescook>
Date:   Mon, 3 Oct 2022 21:04:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 24/39] x86/cet/shstk: Add user-mode shadow stack
 support

On Mon, Oct 03, 2022 at 01:04:37PM -0700, Dave Hansen wrote:
> On 10/3/22 12:43, Kees Cook wrote:
> >> +static inline void set_clr_bits_msrl(u32 msr, u64 set, u64 clear)
> >> +{
> >> +	u64 val, new_val;
> >> +
> >> +	rdmsrl(msr, val);
> >> +	new_val = (val & ~clear) | set;
> >> +
> >> +	if (new_val != val)
> >> +		wrmsrl(msr, new_val);
> >> +}
> > I always get uncomfortable when I see these kinds of generalized helper
> > functions for touching cpu bits, etc. It just begs for future attacker
> > abuse to muck with arbitrary bits -- even marked inline there is a risk
> > the compiler will ignore that in some circumstances (not as currently
> > used in the code, but I'm imagining future changes leading to such a
> > condition). Will you humor me and change this to a macro instead? That'll
> > force it always inline (even __always_inline isn't always inline):
> 
> Oh, are you thinking that this is dangerous because it's so surgical and
> non-intrusive?  It's even more powerful to an attacker than, say
> wrmsrl(), because there they actually have to know what the existing
> value is to update it.  With this helper, it's quite easy to flip an
> individual bit without disturbing the neighboring bits.
> 
> Is that it?

Yeah, it was kind of the combo: both a potential entry point to wrmsrl
for arbitrary values, but also one where all the work is done to mask
stuff out.

> I don't _like_ the #defines, but doing one here doesn't seem too onerous
> considering how critical MSRs are.

I bet there are others, but this just weirded me out. I'll live with a
macro, especially since the chance of it mutating in a non-inline is
very small, but I figured I'd mention the idea.

-- 
Kees Cook