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Date:   Tue, 12 Jan 2021 06:15:39 +0000
From:   "Bae, Chang Seok" <>
To:     "Lutomirski, Andy" <>
CC:     Borislav Petkov <>, Andy Lutomirski <>,
        "" <>, x86-ml <>,
        lkml <>,
        "Metzger, Markus T" <>
Subject: Re: gdbserver + fsgsbase kaputt

> On Jan 11, 2021, at 13:06, Andy Lutomirski <> wrote:
>> On Jan 11, 2021, at 12:00 PM, Borislav Petkov <> 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 <>
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.


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