Re: Linux 6.6-rc3 (DEBUG_VIRTUAL is unhappy on x86)

Peter Zijlstra
2023-10-03T16:42:26Z

On Tue, Oct 03, 2023 at 05:36:27PM +0530, Sumit Garg wrote:
> Hi Linus,
>
> On 10/2/23 02:18, Linus Torvalds wrote:
> > On Sun, 1 Oct 2023 at 07:17, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> > > > > Peter Zijlstra (1):
> > > > > x86,static_call: Fix static-call vs return-thunk
> > > > Hello, the commit above caused a crash on x86 kernel with
> > > > CONFIG_DEBUG_VIRTUAL=y.
> > > OK, I looked into this a little bit, and it turns out that the problematic
> > > address here is from cleanup_trusted() in
> > > security/keys/trusted-keys/trusted_core.c.
> > > (and it's builtin due to CONFIG_TRUSTED_KEYS=y)
> > >
> > > The function is marked as __exit, so it does not fall within the
> > > 'core kernel text address range,' which is between _stext and _etext
> > > (or between _sinittext and _einittext). and thus __text_poke() thinks that
> > > it's vmalloc/module area.
> > >
> > > I think __text_poke() should be taught that functions marked as __exit
> > > also belong to kernel code just like __init.
> > I think your patch is fine (well, whitespace-damaged, but conceptually good).
> >
> > But I also wonder about that
> >
> > static_call_cond(trusted_key_exit)();
> >
> > in cleanup_trusted(). It seems all kinds of pointless to use static
> > calls for something that is done *once*. That's not an optimization,
> > that's honestly just _stupid_. It costs more to do the rewriting that
> > it does to just do the one dynamic indirect call.
>
> That's true, there isn't any real performance benefit here. It is something
> which I mentioned when I was asked to incorporate it here [1]. However, on
> the flip side I think there are security benefits here. We wouldn't like any
> indirect branch speculation attack to leak the trusted key material contents
> here.

1) retpolines;
2) if you can unload modules, you've got bigger problems.