linux-kernel - Re: gdbserver + fsgsbase kaputt

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Date:   Tue, 12 Jan 2021 08:53:53 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        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>
Subject: Re: gdbserver + fsgsbase kaputt

On Tue, Jan 12, 2021 at 3:39 AM Metzger, Markus T
<markus.t.metzger@...el.com> wrote:
>
> > 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.
>
> 64-bit GDB doesn't support FSGSBASE for 32-bit inferiors and it looks like gdbserver
> might not support FSGSBASE, at all.
>
> I had added support for the former as part of the tests I wrote about a year ago [1]
> but never submitted the patch.  Was the discussion ever concluded?
>
> The general behavior should be that GDB reads a regset, overwrites the registers it
> knows about, and writes it back again to preserve the original values of registers it
> doesn't know about.
>
> When I log the values that are read and written for FSGSBASE, however, it looks like
> ptrace is returning a non-zero GS_BASE on a read and gdbserver is writing zero on
> the next write.

I instrumented the kernel, and I see:

[   26.990644] getreg: gs_base = 0xf7f8e000
[   26.991694] getreg: GS=0x63, GSBASE=0xf7f8e000
[   26.993117] PTRACE_SETREGS
[   26.993813] putreg: change gsbase from 0xf7f8e000 to 0x0
[   26.995134] putreg: write GS=0x63; old GSBASE=0x0
[   26.996235] PTRACE_SETREGS done

That's gdbserver reading GS and GSBASE and then telling the kernel to
set GS to the same value and GSBASE to 0.

I can come up with horrible kernel hacks to try to work around this,
but gdbserver is really giving the kernel bad instructions here.

--Andy