From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Fri, 14 Jul 2017 22:27:18 +0100 Subject: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP In-Reply-To: <20170714140605.GB16687@leverpostej> References: <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> <20170714140605.GB16687@leverpostej> Message-ID: <20170714212717.GB1086@leverpostej> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Fri, Jul 14, 2017 at 03:06:06PM +0100, Mark Rutland wrote: > On Fri, Jul 14, 2017 at 01:27:14PM +0100, Ard Biesheuvel wrote: > > On 14 July 2017 at 11:48, Ard Biesheuvel wrote: > > > On 14 July 2017 at 11:32, Mark Rutland wrote: > > >> On Thu, Jul 13, 2017 at 07:28:48PM +0100, Ard Biesheuvel wrote: > > > >>> OK, so here's a crazy idea: what if we > > >>> a) carve out a dedicated range in the VMALLOC area for stacks > > >>> b) for each stack, allocate a naturally aligned window of 2x the stack > > >>> size, and map the stack inside it, leaving the remaining space > > >>> unmapped > > > >> The logical ops (TST) and conditional branches (TB(N)Z, CB(N)Z) operate > > >> on XZR rather than SP, so to do this we need to get the SP value into a > > >> GPR. > > >> > > >> Previously, I assumed this meant we needed to corrupt a GPR (and hence > > >> stash that GPR in a sysreg), so I started writing code to free sysregs. > > >> > > >> However, I now realise I was being thick, since we can stash the GPR > > >> in the SP: > > >> > > >> sub sp, sp, x0 // sp = orig_sp - x0 > > >> add x0, sp, x0 // x0 = x0 - (orig_sp - x0) == orig_sp > > That comment is off, and should say x0 = x0 + (orig_sp - x0) == orig_sp > > > >> sub x0, x0, #S_FRAME_SIZE > > >> tb(nz) x0, #THREAD_SHIFT, overflow > > >> add x0, x0, #S_FRAME_SIZE > > >> sub x0, sp, x0 > > > > You need a neg x0, x0 here I think > > Oh, whoops. I'd mis-simplified things. > > We can avoid that by storing orig_sp + orig_x0 in sp: > > add sp, sp, x0 // sp = orig_sp + orig_x0 > sub x0, sp, x0 // x0 = orig_sp > < check > > sub x0, sp, x0 // x0 = orig_x0 > sub sp, sp, x0 // sp = orig_sp > > ... which works in a locally-built kernel where I've aligned all the > stacks. FWIW, I've pushed out a somewhat cleaned-up (and slightly broken!) version of said kernel source to my arm64/vmap-stack-align branch [1]. That's still missing the backtrace handling, IRQ stack alignment is broken at least on 64K pages, and there's still more cleanup and rework to do. Thanks, Mark. [1] git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git arm64/vmap-stack-align