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Date:   Mon, 25 Feb 2019 07:36:23 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 Feb 25, 2019, at 3:53 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Mon, Feb 25, 2019 at 11:51:44AM +0100, Peter Zijlstra wrote:
>>> 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.
> 
>> 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. 
> 
> *sigh* SWAPGS
> 
>> *thought*... we could delay the actual set_user_seg() thing until after
>> the get_user_catch(), would that work?
> 
> 
> How horrible / broken is this?
> 
> ---
> 
> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> index 321fe5f5d0e9..67c866943102 100644
> --- 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);        \
> }
> 
> static int ia32_restore_sigcontext(struct pt_regs *regs,
>                   struct sigcontext_32 __user *sc)
> {
> +    u16 pre_gs, pre_fs, pre_ds, pre_es;
> +    u16 cur_gs, cur_fs, cur_ds, cur_es;
>    unsigned int tmpflags, err = 0;
>    void __user *buf;
>    u32 tmp;
> @@ -85,10 +89,10 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>         * the handler, but does not clobber them at least in the
>         * normal case.
>         */
> -        RELOAD_SEG(gs);
> -        RELOAD_SEG(fs);
> -        RELOAD_SEG(ds);
> -        RELOAD_SEG(es);
> +        LOAD_SEG(gs);
> +        LOAD_SEG(fs);
> +        LOAD_SEG(ds);
> +        LOAD_SEG(es);
> 
>        COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
>        COPY(dx); COPY(cx); COPY(ip); COPY(ax);
> @@ -106,6 +110,11 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>        buf = compat_ptr(tmp);
>    } get_user_catch(err);
> 
> +    RELOAD_SEG(gs);
> +    RELOAD_SEG(fs);
> +    RELOAD_SEG(ds);
> +    RELOAD_SEG(es);
> +
>    err |= fpu__restore_sig(buf, 1);
> 
>    force_iret();

I would call this pretty horrible. How about we do it without macros? :)

But yes, deferring the segment load until after the read seems fine to me. Frankly, we could also just copy_from_user the whole thing up front — thus code is not really a serious fast path.

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