Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support

[Will Deacon]

On Mon, Apr 20, 2020 at 03:34:57PM -0700, Kees Cook wrote:
> On Mon, Apr 20, 2020 at 09:54:58PM +0100, Will Deacon wrote:
> > On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote:
> > > +	/*
> > > +	 * Since the compiler chooses a 4 bit alignment for the stack,
> > > +	 * let's save one additional bit (9 total), which gets us up
> > > +	 * near 5 bits of entropy.
> > > +	 */
> > > +	choose_random_kstack_offset(get_random_int() & 0x1FF);
> > 
> > Hmm, this comment doesn't make any sense to me. I mean, I get that 0x1ff
> > is 9 bits, and that is 4+5 but so what?
> 
> Er, well, yes. I guess I was just trying to explain why there were 9
> bits saved here and to document what I was seeing the compiler actually
> doing with the values. (And it serves as a comparison to the x86 comment
> which is explaining similar calculations in the face of x86_64 vs ia32.)
> 
> Would something like this be better?
> 
> /*
>  * Since the compiler uses 4 bit alignment for the stack (1 more than
>  * x86_64), let's try to match the 5ish-bit entropy seen in x86_64,
>  * instead of having needlessly lower entropy. As a result, keep the
>  * low 9 bits.
>  */

Yes, thank you! I was missing the comparison to x86_64 and so the one
"additional" bit didn't make sense to me.

With the new comment:

Acked-by: Will Deacon <will@xxxxxxxxxx>

I'm assuming you're merging this via some other tree, but let me know
if you need anything else from me.

Cheers,

Will