[PATCH 6/6] arm64: module: rework module VA range selection

[Mark Rutland]

On Tue, May 09, 2023 at 03:00:50PM +0100, Mark Rutland wrote:
> On Tue, May 09, 2023 at 01:40:12PM +0200, Ard Biesheuvel wrote:
> > On Tue, 9 May 2023 at 13:15, Mark Rutland <mark.rutland at arm.com> wrote:
> > > +       if (kernel_size >= SZ_2G) {
> > > +               pr_warn("Kernel is too large to support modules (%llu bytes)\n",
> > > +                       kernel_size);
> > > +               return 0;
> > > +       }
> > >
> > >         if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
> > > -               /*
> > > -                * Randomize the module region over a 2 GB window covering the
> > > -                * kernel. This reduces the risk of modules leaking information
> > > -                * about the address of the kernel itself, but results in
> > > -                * branches between modules and the core kernel that are
> > > -                * resolved via PLTs. (Branches between modules will be
> > > -                * resolved normally.)
> > > -                */
> > > -               module_range = SZ_2G - (u64)(_end - _stext);
> > > -               module_alloc_base = max((u64)_end - SZ_2G, (u64)MODULES_VADDR);
> > > +               pr_info("2G module region forced by RANDOMIZE_MODULE_REGION_FULL\n");
> > > +       } else if (kernel_size >= SZ_128M) {
> > 
> > I suppose this bound is somewhat arbitrary? I mean, if kernel_size
> > were SZ_128M-SZ_4K, we'd have the exact same problem, and end up using
> > the 2G region all the same, just with a different diagnostic message?
> 
> That's a fair point, and that's also true for the 2G boundary.
> 
> Since the useful bound is arbitrary, it's probably better to log how many pages
> we could potentially use.
> 
> I'll have a go at doing that instead.

Locally I've modified this to always log the number of pages avaialble for !PLT
and PLT, and removed the 128M and 2G boundary messages, so we'll always get
something like:

[    0.487632] Modules: 2G module region forced by RANDOMIZE_MODULE_REGION_FULL
[    0.487914] Modules: 0 pages potentially available for non-PLT usage
[    0.487997] Modules: 514320 pages potentially available for PLT usage

Does that sound ok?

Thanks,
Mark.