[PATCH] remove AND operation in choose_random_kstack_offset()

[Kees Cook]

On Mon, Jun 17, 2024 at 04:52:15PM +0100, Mark Rutland wrote:
> On Mon, Jun 17, 2024 at 01:37:21PM +0000, Yuntao Liu wrote:
> > Since the offset would be bitwise ANDed with 0x3FF in
> > add_random_kstack_offset(), so just remove AND operation here.
> > 
> > Signed-off-by: Yuntao Liu <liuyuntao12 at huawei.com>
> 
> The comments in arm64 and x86 say that they're deliberately capping the
> offset at fewer bits than the result of KSTACK_OFFSET_MAX() masking the
> value with 0x3FF.
> 
> Maybe it's ok to expand that, but if that's the case the commit message
> needs to explain why it's safe add extra bits (2 on arm64, 3 on s39 and
> x86), and those comments need to be updated accordingly.
> 
> As-is, I do not think this patch is ok.

Yeah, I agree: the truncation is intentional and tuned to the
architecture.

-- 
Kees Cook