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

David Laight
2022-10-04T10:18:14Z

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.

David

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