On 8/3/20 3:23 AM, David Laight wrote: > 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: >>>> >>>> From: "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. Understood. Madhavan
