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

Kees Cook
2020-04-20T22:35:19Z

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.
*/

--
Kees Cook