From: Madhavan T. Venkataraman
> Sent: 02 August 2020 19:55
> To: Andy Lutomirski <l...@kernel.org>
> Cc: Kernel Hardening <kernel-harden...@lists.openwall.com>; Linux API
> <linux-...@vger.kernel.org>;
> linux-arm-kernel <linux-arm-ker...@lists.infradead.org>; Linux FS Devel
> <linux-
> fsde...@vger.kernel.org>; linux-integrity <linux-integr...@vger.kernel.org>;
> LKML <linux-
> ker...@vger.kernel.org>; LSM List <linux-security-mod...@vger.kernel.org>;
> Oleg Nesterov
> <o...@redhat.com>; X86 ML <x...@kernel.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, madve...@linux.microsoft.com wrote:
> >>
> >> From: "Madhavan T. Venkataraman" <madve...@linux.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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