Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1422683AbaD3SbN (ORCPT ); Wed, 30 Apr 2014 14:31:13 -0400 Received: from mail-vc0-f179.google.com ([209.85.220.179]:37162 "EHLO mail-vc0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759024AbaD3SbL (ORCPT ); Wed, 30 Apr 2014 14:31:11 -0400 MIME-Version: 1.0 In-Reply-To: References: <53554CDA.1060806@mentor.com> <5357EABB.3070400@zytor.com> <5357F310.8090600@mentor.com> From: Andy Lutomirski Date: Wed, 30 Apr 2014 11:30:50 -0700 Message-ID: Subject: Re: randomized placement of x86_64 vdso To: Kees Cook Cc: Nathan Lynch , "H. Peter Anvin" , "x86@kernel.org" , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 30, 2014 at 11:13 AM, Kees Cook wrote: > On Wed, Apr 30, 2014 at 10:52 AM, Andy Lutomirski wrote: >> On Wed, Apr 30, 2014 at 10:47 AM, Kees Cook wrote: >>> On Wed, Apr 23, 2014 at 10:06 AM, Nathan Lynch wrote: >>>> On 04/23/2014 11:30 AM, H. Peter Anvin wrote: >>>>> On 04/21/2014 09:52 AM, Nathan Lynch wrote: >>>>>> Hi x86/vdso people, >>>>>> >>>>>> I've been working on adding a vDSO to 32-bit ARM, and Kees suggested I >>>>>> look at x86_64's algorithm for placing the vDSO at a randomized offset >>>>>> above the stack VMA. I found that when the stack top occupies the >>>>>> last slot in the PTE (is that the right term?), the vdso_addr routine >>>>>> returns an address below mm->start_stack, equivalent to >>>>>> (mm->start_stack & PAGE_MASK). For instance if mm->start_stack is >>>>>> 0x7fff3ffffc96, vdso_addr returns 0x7fff3ffff000. >>>>>> >>>>>> Since the address returned is always already occupied by the stack, >>>>>> get_unmapped_area detects the collision and falls back to >>>>>> vm_unmapped_area. This results in the vdso being placed in the >>>>>> address space next to libraries etc. While this is generally >>>>>> unnoticeable and doesn't break anything, it does mean that the vdso is >>>>>> placed below the stack when there is actually room above the stack. >>>>>> To me it also seems uncomfortably close to placing the vdso in the way >>>>>> of downward expansion of the stack. >>>>>> >>>>>> I don't have a patch because I'm not sure what the algorithm should >>>>>> be, but thought I would bring it up as vdso_addr doesn't seem to be >>>>>> behaving as intended in all cases. >>>>>> >>>>> >>>>> If the stack occupies the last possible page, how can you say there is >>>>> "space above the stack"? >>>> >>>> Sorry for being unclear. I probably am getting terminology wrong. What >>>> I'm trying to express is that if the stack top is in the last page of >>>> its last-level page table (which may be the last possible page, but >>>> that's not really the interesting case), vdso_addr returns an address >>>> below mm->start_stack. >>> >>> It seems like this is avoidable, then? From your example, it seems >>> like we lose the separated randomization in this case, but we don't >>> need to? Do you have a suggestion for what could be done to fix this? >> >> I don't understand why the vDSO should be special here. Either the >> standard logic for randomizing the placement of DSOs is good, in which >> case it should be good for the vDSO too, or I think we should fix it >> for everything. > > The issue is specific to the vdso randomizing-near-the-stack code; > regular mmap randomization is operating correctly. The reason for > randomizing stack, vdso, and mmap separately is to avoid correlation > of leaked offsets in one to the other regions. I understand why the offset from the stack to the vDSO should be randomized, and why the offset from the stack to, say, glibc should be randomized. What I don't get is why the offset from glibc to the vDSO should be randomized but the offset from glibc to openssl should be deterministic. Or am I misunderstanding? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/