[kernel-hardening] Re: [PATCH 34/38] arm: Implement thread_struct whitelist for hardened usercopy

Kees Cook
2018-01-11T23:21:09Z

On Thu, Jan 11, 2018 at 2:24 AM, Russell King - ARM Linux
<linux@armlinux.org.uk> wrote:
> On Wed, Jan 10, 2018 at 06:03:06PM -0800, Kees Cook wrote:
>> ARM does not carry FPU state in the thread structure, so it can declare
>> no usercopy whitelist at all.
>
> This comment seems to be misleading. We have stored FP state in the
> thread structure for a long time - for example, VFP state is stored
> in thread->vfpstate.hard, so we _do_ have floating point state in
> the thread structure.
>
> What I think this commit message needs to describe is why we don't
> need a whitelist _despite_ having FP state in the thread structure.
>
> At the moment, the commit message is making me think that this patch
> is wrong and will introduce a regression.

Yeah, I will improve this comment; it's not clear enough. The places
where I see state copied to/from userspace are all either static sizes
or already use bounce buffers (or both). e.g.:

err |= __copy_from_user(&hwstate->fpregs, &ufp->fpregs,
sizeof(hwstate->fpregs));

I will adjust the commit log and comment to more clearly describe the
lack of whitelisting due to all-static sized copies.

Thanks!

-Kees

--
Kees Cook
Pixel Security