Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754404AbdGNMzo (ORCPT ); Fri, 14 Jul 2017 08:55:44 -0400 Received: from mail-it0-f49.google.com ([209.85.214.49]:36906 "EHLO mail-it0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754080AbdGNMzm (ORCPT ); Fri, 14 Jul 2017 08:55:42 -0400 MIME-Version: 1.0 In-Reply-To: <20170714125214.GA16687@leverpostej> References: <1499898783-25732-1-git-send-email-mark.rutland@arm.com> <1499898783-25732-7-git-send-email-mark.rutland@arm.com> <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> <20170714125214.GA16687@leverpostej> From: Ard Biesheuvel Date: Fri, 14 Jul 2017 13:55:31 +0100 Message-ID: Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP To: Mark Rutland Cc: Kernel Hardening , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Takahiro Akashi , Catalin Marinas , Dave Martin , James Morse , Laura Abbott , Will Deacon , Kees Cook Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2685 Lines: 57 On 14 July 2017 at 13:52, Mark Rutland wrote: > On Fri, Jul 14, 2017 at 11:48:20AM +0100, 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: >> >> On 13 July 2017 at 18:55, Mark Rutland wrote: >> >> > On Thu, Jul 13, 2017 at 05:10:50PM +0100, Mark Rutland wrote: >> >> >> On Thu, Jul 13, 2017 at 12:49:48PM +0100, Ard Biesheuvel wrote: >> >> >> > On 13 July 2017 at 11:49, Mark Rutland wrote: >> >> >> > > On Thu, Jul 13, 2017 at 07:58:50AM +0100, Ard Biesheuvel wrote: >> >> >> > >> On 12 July 2017 at 23:33, Mark Rutland wrote: >> > This means that we have to align the initial task, so the kernel Image >> > will grow by THREAD_SIZE. Likewise for IRQ stacks, unless we can rework >> > things such that we can dynamically allocate all of those. >> > >> >> We can't currently do that for 64k pages, since the segment alignment >> is only 64k. But we should be able to patch that up I think > > I was assuming that the linked would bump up the segment alignment if a > more-aligned object were placed inside. I guess that doesn't happen in > all cases? > > ... or do you mean when the EFI stub relocates the kernel, assuming > relaxed alignment constraints? > No, I mean under KASLR, which randomizes at SEGMENT_ALIGN granularity. >> >> >> I believe that determining whether the exception was caused by a stack >> >> >> overflow is not something we can do robustly or efficiently. >> >> >> >> Actually, if the stack pointer is within S_FRAME_SIZE of the base, and >> >> the faulting address points into the guard page, that is a pretty >> >> strong indicator that the stack overflowed. That shouldn't be too >> >> costly? >> > >> > Sure, but that's still a a heuristic. For example, that also catches an >> > unrelated vmalloc address gone wrong, while SP was close to the end of >> > the stack. >> >> Yes, but the likelihood that an unrelated stray vmalloc access is >> within 16 KB of a stack pointer that is close ot its limit is >> extremely low, so we should be able to live with the risk of >> misidentifying it. > > I guess, but at that point, why bother? > > That gives us a fuzzy check for one specific "stack overflow", while not > catching the general case. > > So long as we have a reliable stack trace, we can figure out that was > the case, and we don't set the expectation that we're trying to > categorize the general case (minefield and all). > Yes. As long as the context is described accurately, there is no need to make any inferences on behalf of the user.