Re: [PATCH v5 15/30] arm64/sve: Signal handling support

Will Deacon
2017-12-07T10:49:48Z

Hi Kees,

On Wed, Dec 06, 2017 at 11:56:50AM -0800, Kees Cook wrote:
> On Tue, Oct 31, 2017 at 8:51 AM, Dave Martin <Dave.Martin@arm.com> wrote:
> > Miscellaneous:
> >
> > * Change inconsistent copy_to_user() calls to __copy_to_user() in
> > preserve_sve_context().
> >
> > There are already __put_user_error() calls here.
> >
> > The whole extended signal frame is already checked for
> > access_ok(VERIFY_WRITE) in get_sigframe().
>
> Verifying all these __copy_to/from_user() calls is rather non-trivial.
> For example, I had to understand that the access_ok() check actually
> spans memory that both user->sigframe and user->next_frame point into.

I don't think that's particularly difficult -- you just have to read the
four lines preceding the access_ok.

> And it isn't clear to me that all users of apply_user_offset() are
> within this range too, along with other manually calculated offsets in
> setup_sigframe().

The offsets passed into apply_user_offset are calculated by
setup_sigframe_layout as the stack is allocated, so they're correct by
construction. We could add a size check in apply_user_offset if you like?

> And it's not clear if parse_user_sigframe() is safe either. Are
> user->fpsimd and user->sve checked somewhere? It seems like it's
> safely contained by in sf->uc.uc_mcontext.__reserved, but it's hard to
> read, though I do see access_ok() checks against __reserved at the end
> of the while loop.

This one is certainly more difficult to follow, mainly because it's spread
about a bit and we have to check the extra context separately. However, the
main part of the frame is checked in sys_rt_sigreturn before calling
restore_sigframe, and the extra context is checked in parse_user_sigframe
if we find it.

Dave, any thoughts on making this easier to understand?

Will