Re: [PATCH] remove AND operation in choose_random_kstack_offset()

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Jun 18, 2024, at 12:45, Mark Rutland wrote:
> On Mon, Jun 17, 2024 at 10:33:08PM +0200, Arnd Bergmann wrote:
>> On Mon, Jun 17, 2024, at 20:22, Kees Cook wrote:
>> > On Mon, Jun 17, 2024 at 04:52:15PM +0100, Mark Rutland wrote:

> Sorry, to be clear, I'm happy for this to change, so long as:
>
> * The commit message explains why that's safe.
>
>   IIUC this goes from 511 to 1023 bytes on arm64, which is ~3% of the
>   stack, so maybe that is ok. It'd be nice to see any rationale/analysis
>   beyond "the offset would be bitwise ANDed with 0x3FF".

Absolutely agreed, and the commit message should also clarify that
the increase has already happened as an unintended side-effect
of commit 9c573cd31343 ("randomize_kstack: Improve entropy
diffusion").

> * The comments in architecture code referring to the masking get
>   removed/updated along with the masking.

Right.

FWIW, I also wouldn't mind to having a compile-time option
that configures the number of random bits on the stack offset,
but my preference here is to have a reasonable default and
not need a config option.

    Arnd