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

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Date:   Sun, 1 Oct 2023 13:48:21 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Kaplan <David.Kaplan@....com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        x86@...nel.org
Subject: Re: Linux 6.6-rc3 (DEBUG_VIRTUAL is unhappy on x86)

On Sun, 1 Oct 2023 at 07:17, Hyeonggon Yoo <42.hyeyoo@...il.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.

Side note: the same is true of the init-time call, which does

                static_call_update(trusted_key_init,
                                   trusted_key_sources[i].ops->init);
                ...
                ret = static_call(trusted_key_init)();

which again is a *lot* more expensive than just doing the indirect
function call.

So while I don't think your patch is wrong, I do think that the cause
here is plain silly code, and that trusted key code simply should not
do the crazy thing it does (and that causes silly problems).

               Linus