linux-kernel - Re: [RFC PATCH 4/6] uprobes/x86: Emulate rip-relative call's

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140410141806.GA23997@redhat.com>
Date:	Thu, 10 Apr 2014 16:18:06 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <aarapov@...hat.com>,
	David Long <dave.long@...aro.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jonathan Lebon <jlebon@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/6] uprobes/x86: Emulate rip-relative call's

On 04/10, Denys Vlasenko wrote:
>
> There is this monstrosity, "16-bit override for branches" in 64-mode:
>
> 66 e8 nn nn       callw   <offset16>
>
> Nobody sane uses it because it truncates instruction pointer.
>
> Or rather, *I think* it should truncate it (i.e. zero-extend to full width),
> but conceivably some CPUs can be buggy wrt that:
> they can decide to modify only lower 16 bits of IP,
> or even they can decided to use signed <offset16> but apply it
> to full-width RIP.
>
> AMD manuals are not clear on what exactly should happen.
>
> I am sure no one sane uses this form of branch instructions
> in 32-bit and 64-bit code.
>
> I don't think we should be trying to support it "correctly"
> (we can just let program crash with SIGILL or something),
> we only need to make sure we don't overlook its existence
> and thus are not tricked into touching or modifying unrelated data.

And after the quick check it seems that lib/insn.c doesn't parse
"66 e8 nn nn" correctly. It treats the next 2 bytes as the part
of 32bit offset.

> Imagine that   66 e8 nn nn    bytes are exactly at the end of
> a page,

this doesn't matter. We always read MAX_UINSN_BYTES bytes, so

> and we wrongly assume that offset is 32-bit, not 16-bit.
> 66 e8 nn nn.

see above this will always happen.

So I think branch_setup_xol_ops() should simply return -ENOSYS in
this case, and let the default_ code execute it out of line. Given
that nobody should use this insn, this is probably not too bad.

We can teach branch_xol_ops to handle this insn correctly later.

Now the question is, how I can detect this insn correctly? I mean,
using the "right" helpers from lib/insn.c ?

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/