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

[Kees Cook]

On Tue, Oct 04, 2022 at 10:17:57AM +0000, David Laight wrote:
> From: Dave Hansen
> > Sent: 03 October 2022 21:05
> > 
> > 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?
> > 
> > I don't _like_ the #defines, but doing one here doesn't seem too onerous
> > considering how critical MSRs are.
> 
> How often is the 'msr' number not a compile-time constant?
> Adding rd/wrmsr variants that verify this would reduce the
> attack surface as well.

Oh, yes! I do this all the time with FORTIFY shenanigans. Right, so,
instead of a macro, the "cannot be un-inlined" could be enforced with
this (untested):

static __always_inline void set_clr_bits_msrl(u32 msr, u64 set, u64 clear)
{
	u64 val, new_val;

	BUILD_BUG_ON(!__builtin_constant_p(msr) ||
		     !__builtin_constant_p(set) ||
		     !__builtin_constant_p(clear));

	rdmsrl(msr, val);
	new_val = (val & ~clear) | set;

	if (new_val != val)
		wrmsrl(msr, new_val);
}

-- 
Kees Cook