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. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
