linux-kernel - Re: gdbserver + fsgsbase kaputt

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Message-ID: <CALCETrV=BpFwR-RU5ORioBCZj3RwK7nmD2Yz3VNd4gfFjukRAw@mail.gmail.com>
Date:   Mon, 11 Jan 2021 15:40:19 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>, tdevries@...e.com,
        x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: gdbserver + fsgsbase kaputt

On Mon, Jan 11, 2021 at 1:06 PM Andy Lutomirski <luto@...capital.net> wrote:
>
>
> > On Jan 11, 2021, at 12:00 PM, Borislav Petkov <bp@...en8.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?