linux-kernel - RE: gdbserver + fsgsbase kaputt

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Date:   Wed, 20 Jan 2021 15:42:34 +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

> From: Andy Lutomirski <luto@...nel.org>
> 
> On Tue, Jan 12, 2021 at 9:02 AM Metzger, Markus T
> <markus.t.metzger@...el.com> wrote:
> >
> > > [   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.
> 
> Indeed.  But we have this pesky no-userspace-regressions policy in the kernel.
> 
> So the question I have is: is this enough of a regression that we need
> to hack around it in the kernel?  The specific broken use case seems
> quite niche: 64-bit gdbserver targeting 32-bit userspace.  It's taken
> two-and-a-half kernel releases for anyone to notice, because sensible
> people use plain gdb for local debugging and gdbserver for debugging
> VMs, embedded targets, and such.

IMHO I'd just fix GDB and leave it at that.  The kernel changes exposed a bug
 in gdbserver.  Tom already submitted a fix.

I'm wondering why we're considering some ugly hacks in the kernel for what's
obviously a bug in user-space, yet ignore changes to GDB functionality (for changing
FS/GS from within GDB) we had been discussing a year ago.

Regards,
Markus.
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