linux-kernel - RE: gdbserver + fsgsbase kaputt

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Date:   Tue, 12 Jan 2021 17:02:02 +0000
From:   "Metzger, Markus T" <markus.t.metzger@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        "tdevries@...e.com" <tdevries@...e.com>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: RE: gdbserver + fsgsbase kaputt

> [   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.

I agree that this looks like a GDB bug rather than a kernel bug.  GDB
should preserve the GS_BASE value if it doesn't intend to change it.

Markus.
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