linux-kernel - Re: [RFC][PATCH] objtool: STAC/CLAC validation

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Message-ID: <20190225105144.GJ32477@hirez.programming.kicks-ass.net>
Date:   Mon, 25 Feb 2019 11:51:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Julien Thierry <julien.thierry@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-arm-kernel@...ts.infradead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [RFC][PATCH] objtool: STAC/CLAC validation

On Fri, Feb 22, 2019 at 03:55:25PM -0800, Andy Lutomirski wrote:
> I'm wondering if we can just change the code that does getreg() and
> load_gs_index() so it doesn't do it with AC set.  Also, what about
> paravirt kernels?  They'll call into PV code for load_gs_index() with
> AC set.

Paravirt can go bugger off. There's no sane way to fix that.

Luckily the load_gs_index() thing is part of the paravirt me harder crap
and so nobody sane should ever hit that.

I don't fully understand that code at all; I also have no clue why GS
has paravirt bits on but the other segments do not. But it looks like we
want to do that RELOAD_SEG() crap under SMAP because of the GET_SEG() ->
get_user_ex() thing.

Anyway, I only see 3 options here:

 1) live with the paravirt me harder builds complaining
 2) exclude the AC validation from the paravirt me harder builds
 3) rewrite this code to no need that stupid call in the first place


2 seems like an exceptionally bad ideal, 3 would need someone that
understands this, so for now I'll pick 1 :-)


*thought*... we could delay the actual set_user_seg() thing until after
the get_user_catch(), would that work?