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Date:   Mon, 11 Jan 2021 21:00:27 +0100
From:   Borislav Petkov <>
To:     Andy Lutomirski <>
Cc:     "Chang S. Bae" <>,,
        x86-ml <>, lkml <>
Subject: Re: gdbserver + fsgsbase kaputt

On Mon, Jan 11, 2021 at 11:27:38AM -0800, Andy Lutomirski wrote:
> Hmm.  Can you try booting with unsafe_fsgsbase and bisecting further?

Well, that bisection ended in that patch:

# first bad commit: [b745cfba44c152c34363eea9e052367b6b1d652b] x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit

so I can't go further.

Or do you mean I should add "unsafe_fsgsbase" to grub cmdline and bisect
with fsgsbase enabled in all test kernels?

> And maybe send me your test binary?

It is trivial:

main (void)
  return 0;

how can that make any difference or are you thinking compiler differences?

Lemme send it to you.

> I tried to reproduce this, but it worked fine, even if I compile the
> test program with -fstack-protector-all.


> Off the top of my head, I would have expected this to fix it:
> commit 40c45904f818c1f6555294ca27afc5fda4f09e68
> Author: Andy Lutomirski <>
> Date:   Fri Jun 26 10:24:29 2020 -0700
>     x86/ptrace: Fix 32-bit PTRACE_SETREGS vs fsbase and gsbase

More hmm.


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