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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Mon, 25 Feb 2019 07:41:50 -0800
Message-ID: <CALCETrW-p4iF-RMecyNt1JDwKh74c_tw67FCW7Yr_Fd3adtGUw@mail.gmail.com>
Subject: Re: [PATCH 2/6] x86/ia32: Fix ia32_restore_sigcontext AC leak
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>, valentin.schneider@arm.com,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Feb 25, 2019 at 4:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Don't call load_gs_index() with AC set; delay the segment setting
> until after the AC section.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/ia32/ia32_signal.c |   29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
>
> --- a/arch/x86/ia32/ia32_signal.c
> +++ b/arch/x86/ia32/ia32_signal.c
> @@ -60,17 +60,21 @@
>         regs->seg = GET_SEG(seg) | 3;                   \
>  } while (0)
>
> -#define RELOAD_SEG(seg)                {               \
> -       unsigned int pre = GET_SEG(seg);        \
> -       unsigned int cur = get_user_seg(seg);   \
> -       pre |= 3;                               \
> -       if (pre != cur)                         \
> -               set_user_seg(seg, pre);         \
> +#define LOAD_SEG(seg)          {                       \
> +       pre_##seg = 3 | GET_SEG(seg);                   \
> +       cur_##seg = get_user_seg(seg);                  \
> +}
> +
> +#define RELOAD_SEG(seg)                {                       \
> +       if (pre_##seg != cur_##seg)                     \
> +               set_user_seg(seg, pre_##seg);           \
>  }

This is so tangled.

How about changing RELOAD_SEG to replace unsigned int pre =
GET_SEG(seg); with unsigned int pre = (seg); to make it less magic.
Then do:

unsigned int gs = GET_SEG(gs);

...

RELOAD_SEG(gs);

And now the code actually does what it looks like it does.