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Date:   Mon, 3 Oct 2022 21:04:00 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
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        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
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        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v2 24/39] x86/cet/shstk: Add user-mode shadow stack
 support
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 <202210031203.EB0DC0B7DD@keescook>
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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