linux-kernel - RE: gdbserver + fsgsbase kaputt

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Message-ID: <DM5PR11MB1690CB5004CADCE5E9D5A221DEAA0@DM5PR11MB1690.namprd11.prod.outlook.com>
Date:   Tue, 12 Jan 2021 11:39:07 +0000
From:   "Metzger, Markus T" <markus.t.metzger@...el.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "Lutomirski, Andy" <luto@...capital.net>
CC:     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

> 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.

Chang, is that also what you were seeing?

Regards,
Markus.

[1]
https://lkml.org/lkml/2019/11/29/306

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