linux-kernel - Re: [PATCH 1/1] x86/entry/64: randomize kernel stack offset upon syscall

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Date:   Wed, 10 Apr 2019 07:48:55 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Elena Reshetova <elena.reshetova@...el.com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 1/1] x86/entry/64: randomize kernel stack offset upon syscall

On Wed, Apr 10, 2019 at 4:43 AM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Elena Reshetova <elena.reshetova@...el.com> wrote:
>
> > 2)  Andy's tests, misc-tests: ./timing_test_64 10M sys_enosys
> >     base:                                        10000000 loops in 1.62224s = 162.22 nsec / loop
> >     random_offset (prandom_u32() every syscall): 10000000 loops in 1.64660s = 166.26 nsec / loop
>
> Stupid question, how did you manage to buil timing_test_64? Here it fails
> with a bog standard gcc 7.3.0 x86-64 distro toolchain:
>
>  dagon:~/luto-misc-tests.git> make timing_test_64
>  g++ -m64 -o timing_test_64 -O2 -g -std=gnu++11 -pthread -Wall  timing_test.cc -lrt -ldl
>  /usr/bin/ld: /tmp/cc8VRkuV.o: relocation R_X86_64_32S against `.text.startup' can not be used when making a PIE object; recompile with -fPIC
>  /usr/bin/ld: final link failed: Nonrepresentable section on outputcollect2: error: ld returned 1 exit status
>  Makefile:39: recipe for target 'timing_test_64' failed
>

I think your toolchain is screwy.  If I create this file as ingo.c:

#include <stdio.h>

int main(int argc, char **argv)
{
    printf("Hello world!");

    return 0;
}

And build it like this, it fails:

$ gcc -o ingo -g ingo.c -pie
/usr/bin/ld: /tmp/ccofYU9N.o: relocation R_X86_64_32 against `.rodata'
can not be used when making a PIE object; recompile with -fPIC
/usr/bin/ld: final link failed: nonrepresentable section on output
collect2: error: ld returned 1 exit status

Which I assume means that -pie requires -fPIC, and your toolchain is
screwed up and is defaulting to useless options.  I'm guessing you
should file a bug against your distro gcc package.  For me, it works
if I remove -pie.