linux-kernel - Re: gdbserver + fsgsbase kaputt

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Date:   Tue, 12 Jan 2021 06:15:39 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     "Lutomirski, Andy" <luto@...capital.net>
CC:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        "tdevries@...e.com" <tdevries@...e.com>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>
Subject: Re: gdbserver + fsgsbase kaputt


> On Jan 11, 2021, at 13:06, 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.

I was able to find this patch in that way:

commit 0bf7e460361c703333f3a82e50e7871465fe20f9
Author: Andy Lutomirski <luto@...nel.org>
Date:   Thu May 28 16:13:51 2020 -0400

    x86/process/64: Use FSBSBASE in switch_to() if available


The GDB behavior looks to be different between the two cases -- with vs
without gdb server, when I checked the GS/GSBASE values on the ptrace front.

It set the correct GSBASE (e.g.,=0xf7fcf0c0) of GS=0x63 without running the
server. But GSBASE=0 with the server. When I forced to set the correct base,
it exited normally.

Thanks,
Chang