Date: Fri, 14 Jul 2017 22:27:18 +0100 From: Mark Rutland <mark.rutland@....com> To: Ard Biesheuvel <ard.biesheuvel@...aro.org> Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Takahiro Akashi <akashi.takahiro@...aro.org>, Catalin Marinas <catalin.marinas@....com>, Dave Martin <dave.martin@....com>, James Morse <james.morse@....com>, Laura Abbott <labbott@...oraproject.org>, Will Deacon <will.deacon@....com>, Kees Cook <keescook@...omium.org> Subject: Re: Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP 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 <ard.biesheuvel@...aro.org> wrote: > > > On 14 July 2017 at 11:32, Mark Rutland <mark.rutland@....com> 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 . 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.  git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git arm64/vmap-stack-align
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