[PATCH] remove AND operation in choose_random_kstack_offset()

[Mark Rutland]

Hi Arnd,

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:
> >> 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.
> 
> It may be intentional, but it's clearly nonsense: there is nothing
> inherent to the architecture that means we have can go only 256
> bytes instead of 512 bytes into the 16KB available stack space.
> 
> As far as I can tell, any code just gets bloated to the point
> where it fills up the available memory, regardless of how
> much you give it. I'm sure one can find code paths today that
> exceed the 16KB, so there is no point pretending that 15.75KB
> is somehow safe to use while 15.00KB is not.
> 
> I'm definitely in favor of making this less architecture
> specific, we just need to pick a good value, and we may well
> end up deciding to use less than the default 1KB. We can also
> go the opposite way and make the limit 4KB but then increase
> the default stack size to 20KB for kernels that enable
> randomization.

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".

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

My complaint was that the patch didn't do those things.

Mark.