linux-kernel - Re: [PATCH] x86/smap: Fix the smap

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Message-ID: <20200917060432.GA31960@zn.tnic>
Date:   Thu, 17 Sep 2020 08:04:32 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Bill Wendling <morbo@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Thelen <gthelen@...gle.com>,
        John Sperbeck <jsperbeck@...gle.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86/smap: Fix the smap_save() asm

On Wed, Sep 16, 2020 at 11:48:42PM +0100, Andrew Cooper wrote:
> Every day is a school day.

Tell me about it...

> This is very definitely one to be filed in obscure x86 corner cases.
> 
> The code snippet above is actually wrong for the kernel, as it uses one
> slot of the red-zone.  Recompiling with -mno-red-zone makes something
> which looks safe stack-wise, give or take this behaviour.

Right, we recently disabled red zone in the early decompression stage,
for SEV-ES:

https://git.kernel.org/tip/6ba0efa46047936afa81460489cfd24bc95dd863

I probably should go audit that for similar funnies:

$ objdump -d arch/x86/boot/compressed/vmlinux | grep -E "pop.*\(%[er]?sp"
$

Nope, nothing. Because building your snippet with:

$ gcc -Wall -O2 -mno-red-zone -o flags{,.c}

still does use that one slot:

0000000000001050 <main>:
    1050:       48 83 ec 18             sub    $0x18,%rsp
    1054:       48 8d 3d a9 0f 00 00    lea    0xfa9(%rip),%rdi        # 2004 <_IO_stdin_used+0x4>
    105b:       31 c0                   xor    %eax,%eax
    105d:       9c                      pushfq
    105e:       8f 44 24 08             popq   0x8(%rsp)
    1062:       48 8b 74 24 08          mov    0x8(%rsp),%rsi

Wonder if that flag -mno-red-zone even does anything...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette