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Date:   Tue, 12 Jan 2021 17:02:02 +0000
From:   "Metzger, Markus T" <>
To:     Andy Lutomirski <>
CC:     "Bae, Chang Seok" <>,
        Borislav Petkov <>,
        "" <>, x86-ml <>,
        lkml <>
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.

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