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Date: Mon, 3 Aug 2020 08:23:03 +0000
From: David Laight <David.Laight@...LAB.COM>
To: "'Madhavan T. Venkataraman'" <madvenka@...ux.microsoft.com>, "Andy
 Lutomirski" <luto@...nel.org>
CC: Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux API
	<linux-api@...r.kernel.org>, linux-arm-kernel
	<linux-arm-kernel@...ts.infradead.org>, Linux FS Devel
	<linux-fsdevel@...r.kernel.org>, linux-integrity
	<linux-integrity@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, "LSM
 List" <linux-security-module@...r.kernel.org>, Oleg Nesterov
	<oleg@...hat.com>, X86 ML <x86@...nel.org>
Subject: RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

From: Madhavan T. Venkataraman
> Sent: 02 August 2020 19:55
> To: Andy Lutomirski <luto@...nel.org>
> Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>; Linux API <linux-api@...r.kernel.org>;
> linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>; Linux FS Devel <linux-
> fsdevel@...r.kernel.org>; linux-integrity <linux-integrity@...r.kernel.org>; LKML <linux-
> kernel@...r.kernel.org>; LSM List <linux-security-module@...r.kernel.org>; Oleg Nesterov
> <oleg@...hat.com>; X86 ML <x86@...nel.org>
> Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor
> 
> More responses inline..
> 
> On 7/28/20 12:31 PM, Andy Lutomirski wrote:
> >> On Jul 28, 2020, at 6:11 AM, madvenka@...ux.microsoft.com wrote:
> >>
> >> ´╗┐From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
> >>
> >
> > 2. Use existing kernel functionality.  Raise a signal, modify the
> > state, and return from the signal.  This is very flexible and may not
> > be all that much slower than trampfd.
> 
> Let me understand this. You are saying that the trampoline code
> would raise a signal and, in the signal handler, set up the context
> so that when the signal handler returns, we end up in the target
> function with the context correctly set up. And, this trampoline code
> can be generated statically at build time so that there are no
> security issues using it.
> 
> Have I understood your suggestion correctly?

I was thinking that you'd just let the 'not executable' page fault
signal happen (SIGSEGV?) when the code jumps to on-stack trampoline
is executed.

The user signal handler can then decode the faulting instruction
and, if it matches the expected on-stack trampoline, modify the
saved registers before returning from the signal.

No kernel changes and all you need to add to the program is
an architecture-dependant signal handler.

	David

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