RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

[David Laight]

From: Madhavan T. Venkataraman
> Sent: 02 August 2020 19:55
> To: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>; Linux API <linux-api@xxxxxxxxxxxxxxx>;
> linux-arm-kernel <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx>; Linux FS Devel <linux-
> fsdevel@xxxxxxxxxxxxxxx>; linux-integrity <linux-integrity@xxxxxxxxxxxxxxx>; LKML <linux-
> kernel@xxxxxxxxxxxxxxx>; LSM List <linux-security-module@xxxxxxxxxxxxxxx>; Oleg Nesterov
> <oleg@xxxxxxxxxx>; X86 ML <x86@xxxxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxxx wrote:
> >>
> >> ïFrom: "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx>
> >>
> >
> > 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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