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

Kees Cook
2022-10-04T04:04:13Z

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