Date: Fri, 14 Jul 2017 15:39:39 +0100 From: Robin Murphy <robin.murphy@....com> To: Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org> Cc: Kees Cook <keescook@...omium.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, James Morse <james.morse@....com>, Takahiro Akashi <akashi.takahiro@...aro.org>, Dave Martin <dave.martin@....com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Laura Abbott <labbott@...oraproject.org> Subject: Re: Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP On 14/07/17 15:06, 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 Haven't you now forcibly cleared the top bit of x0 thanks to overflow? Robin. > sub sp, sp, x0 // sp = orig_sp > > ... which works in a locally-built kernel where I've aligned all the > stacks. > >> ... only, this requires a dedicated stack region, and so we'd need to >> check whether sp is inside that window as well. >> >> The easieast way would be to use a window whose start address is base2 >> aligned, but that means the beginning of the kernel VA range (where >> KASAN currently lives, and cannot be moved afaik), or a window at the >> top of the linear region. Neither look very appealing >> >> So that means arbitrary low and high limits to compare against in this >> entry path. That means more GPRs I'm afraid. > > Could you elaborate on that? I'm not sure that I follow. > > My understanding was that the comprimise with this approach is that we > only catch overflow/underflow within THREAD_SIZE of the stack, and can > get false-negatives elsewhere. Otherwise, IIUC this is sufficient > > Are you after a more stringent check (like those from the two existing > proposals that caught all out-of-bounds accesses)? > > Or am I missing something else? > > Thanks, > Mark. > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@...ts.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel >
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