linux-kernel - Re: objtool clac/stac handling change..

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Message-ID: <CAHk-=wicOPQwuDUzFyDTBgr4UvQJHPdCX7_6BLaK6cve6CqBSg@mail.gmail.com>
Date:   Thu, 2 Jul 2020 12:52:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christophe Leroy <christophe.leroy@....fr>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: objtool clac/stac handling change..

On Thu, Jul 2, 2020 at 6:32 AM Michael Ellerman <mpe@...erman.id.au> wrote:
>
> Probably the simplest option for us is to just handle it in our
> unsafe_op_wrap(). I'll try and come up with something tomorrow.

IMy suggestion was to basically just always handle it in all exception cases.

And note that IU don't mean the fault handler: obviously page faults
(or unaligned faults or whatever) can happen while in a user access
region.

But I mean any time fixup_exception() triggers.

For x86, this is in fact particularly natural: it involves just always
clearing the AC bit in the "struct pt_regs" that fixup_exception()
gets anyway. We can do it without even bothering with checking for
CLAC/STAC support, since without it, AC is meaningless in kernel mode
anyway, but also because doing "user_access_end()" in the exception
would be pointless: AC is restored by the exception routine, so on x86
you *have* to do it by just modifying the return state.

             Linus