linux-kernel - Re: [PATCH] x86: Disable CET instrumentation in the kernel

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Message-ID: <20210129170755.GF27841@zn.tnic>
Date:   Fri, 29 Jan 2021 18:07:55 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nikolay Borisov <nborisov@...e.com>, x86@...nel.org,
        Masami Hiramatsu <masami.hiramatsu@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Seth Forshee <seth.forshee@...onical.com>
Subject: Re: [PATCH] x86: Disable CET instrumentation in the kernel

On Fri, Jan 29, 2021 at 11:03:31AM -0600, Josh Poimboeuf wrote:
> On Fri, Jan 29, 2021 at 06:54:08PM +0200, Nikolay Borisov wrote:
> > 
> > 
> > On 29.01.21 г. 18:49 ч., Josh Poimboeuf wrote:
> > > Agreed, stable is a good idea.   I think Nikolay saw it with GCC 9.
> > 
> > 
> > Yes I did, with the default Ubuntu compiler as well as the default gcc-10 compiler: 
> > 
> > # gcc -v -Q -O2 --help=target | grep protection
> > 
> > gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04) 
> > COLLECT_GCC_OPTIONS='-v' '-Q' '-O2' '--help=target' '-mtune=generic' '-march=x86-64'
> >  /usr/lib/gcc/x86_64-linux-gnu/9/cc1 -v -imultiarch x86_64-linux-gnu help-dummy -dumpbase help-dummy -mtune=generic -march=x86-64 -auxbase help-dummy -O2 -version --help=target -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -fcf-protection -o /tmp/ccSecttk.s
> > GNU C17 (Ubuntu 9.3.0-17ubuntu1~20.04) version 9.3.0 (x86_64-linux-gnu)
> > 	compiled by GNU C version 9.3.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.22.1-GMP
> > 
> > 
> > It has -fcf-protection turned on by default it seems. 
> 
> Yup, explains why I didn't see it:
> 
> gcc version 10.2.1 20201125 (Red Hat 10.2.1-9) (GCC)
> COLLECT_GCC_OPTIONS='-v' '-Q' '-O2' '--help=target' '-mtune=generic' '-march=x86-64'
>  /usr/libexec/gcc/x86_64-redhat-linux/10/cc1 -v help-dummy -dumpbase help-dummy -mtune=generic -march=x86-64 -auxbase help-dummy -O2 -version --help=target -o /tmp/cclBz55H.s

The fact that you triggered it with an Ubuntu gcc explains why the
original patch adding that switch:

29be86d7f9cb ("kbuild: add -fcf-protection=none when using retpoline flags")

came from a Canonical.

Adding the author to Cc for FYI.

Seth, you can find this thread starting here:

https://lkml.kernel.org/r/20210128215219.6kct3h2eiustncws@treble

Thx.

-- 
Regards/Gruss,
    Boris.

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