Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization

[Kees Cook]

On Thu, Mar 07, 2024 at 12:10:34PM +0100, Arnd Bergmann wrote:
> There is not even any attempt to use the most random bits of
> the cycle counter, as both the high 22 to 24 bits get masked
> out (to keep the wasted stack space small) and the low 3 or 4
> bits get ignored because of stack alignment. If there was
> any desire to make it more random, a trivial improvement
> would be:
> 
> +++ b/include/linux/randomize_kstack.h
> @@ -80,7 +80,7 @@ DECLARE_PER_CPU(u32, kstack_offset);
>         if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
>                                 &randomize_kstack_offset)) {            \
>                 u32 offset = raw_cpu_read(kstack_offset);               \
> -               offset ^= (rand);                                       \
> +               offset = ror32(offset, 5) & (rand);                     \

Shouldn't this stay ^ instead of & ?

>                 raw_cpu_write(kstack_offset, offset);                   \
>         }                                                               \
>  } while (0)

But yeah, we should likely make this change regardless.

> My impression is that is is already bordering on becoming
> a "bespoke rng" implementation that Jason was objecting to,
> so the current version is intentionally left weak in order
> to not even give the appearance of being a security relevant
> feature.

I don't think it's bad to make a trivial improvement to entropy diffusion.

-- 
Kees Cook