Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210111200027.GH25645@zn.tnic> <E74AC970-CFCF-4CFD-A71F-F719F5BCE2DC@amacapital.net>
In-Reply-To: <E74AC970-CFCF-4CFD-A71F-F719F5BCE2DC@amacapital.net>
From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 11 Jan 2021 15:40:19 -0800
Message-ID: <CALCETrV=BpFwR-RU5ORioBCZj3RwK7nmD2Yz3VNd4gfFjukRAw@mail.gmail.com>
Subject: Re: gdbserver + fsgsbase kaputt
To:     Borislav Petkov <bp@alien8.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        "Chang S. Bae" <chang.seok.bae@intel.com>, tdevries@suse.com,
        x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Mon, Jan 11, 2021 at 1:06 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
>
> > On Jan 11, 2021, at 12:00 PM, Borislav Petkov <bp@alien8.de> wrote:
> >
>
>
> > Or do you mean I should add "unsafe_fsgsbase" to grub cmdline and bisect
> > with fsgsbase enabled in all test kernels?
>
> Yes. But I can also look myself in a bit.
>

Tom, if I reproduce it in an interactive gdb and play a bit, I get:

Program received signal SIGSEGV, Segmentation fault.
0xf7df2cb6 in init_cacheinfo () from target:/lib/libc.so.6
(gdb) p $gs = $gs
$1 = 99
(gdb) si

Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.

That's gdb itself crashing.  Any idea what's wrong?