From: Andy Lutomirski <luto@kernel.org>
To: "Metzger, Markus T" <markus.t.metzger@intel.com>
Cc: "Bae, Chang Seok" <chang.seok.bae@intel.com>,
Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
"tdevries@suse.com" <tdevries@suse.com>, x86-ml <x86@kernel.org>,
lkml <linux-kernel@vger.kernel.org>
Subject: Re: gdbserver + fsgsbase kaputt
Date: Tue, 12 Jan 2021 08:53:53 -0800 [thread overview]
Message-ID: <CALCETrUzsx2gpJGwz4fwLTLTKG4i=izaGRYL-3dLrn389JV17w@mail.gmail.com> (raw)
In-Reply-To: <DM5PR11MB1690CB5004CADCE5E9D5A221DEAA0@DM5PR11MB1690.namprd11.prod.outlook.com>
On Tue, Jan 12, 2021 at 3:39 AM Metzger, Markus T
<markus.t.metzger@intel.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
next prev parent reply other threads:[~2021-01-12 16:55 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-11 18:15 gdbserver + fsgsbase kaputt Borislav Petkov
2021-01-11 19:27 ` Andy Lutomirski
2021-01-11 20:00 ` Borislav Petkov
2021-01-11 21:06 ` Andy Lutomirski
2021-01-11 23:40 ` Andy Lutomirski
2021-01-11 23:52 ` Tom de Vries
2021-01-12 3:31 ` Andy Lutomirski
2021-01-12 8:45 ` Tom de Vries
2021-01-12 6:15 ` Bae, Chang Seok
2021-01-12 11:39 ` Metzger, Markus T
2021-01-12 16:53 ` Andy Lutomirski [this message]
2021-01-12 17:02 ` Metzger, Markus T
2021-01-12 17:13 ` Andy Lutomirski
2021-01-20 15:42 ` Metzger, Markus T
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CALCETrUzsx2gpJGwz4fwLTLTKG4i=izaGRYL-3dLrn389JV17w@mail.gmail.com' \
--to=luto@kernel.org \
--cc=bp@alien8.de \
--cc=chang.seok.bae@intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=markus.t.metzger@intel.com \
--cc=tdevries@suse.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).