Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753731AbbK2WGI (ORCPT ); Sun, 29 Nov 2015 17:06:08 -0500 Received: from mail-vk0-f54.google.com ([209.85.213.54]:36503 "EHLO mail-vk0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753493AbbK2WFK convert rfc822-to-8bit (ORCPT ); Sun, 29 Nov 2015 17:05:10 -0500 MIME-Version: 1.0 In-Reply-To: <20151129213300.GA28845@fergus.ozlabs.ibm.com> References: <20151129213300.GA28845@fergus.ozlabs.ibm.com> Date: Sun, 29 Nov 2015 19:05:08 -0300 Message-ID: Subject: Re: [RFC] kvm - possible out of bounds From: =?UTF-8?Q?Geyslan_Greg=C3=B3rio_Bem?= To: Paul Mackerras Cc: Gleb Natapov , Paolo Bonzini , Alexander Graf , Benjamin Herrenschmidt , Michael Ellerman , kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, LKML Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2982 Lines: 96 2015-11-29 18:33 GMT-03:00 Paul Mackerras : > On Sun, Nov 29, 2015 at 05:14:03PM -0300, Geyslan Gregório Bem wrote: >> Hello, >> >> I have found a possible out of bounds reading in >> arch/powerpc/kvm/book3s_64_mmu.c (kvmppc_mmu_book3s_64_xlate >> function). pteg[] array could be accessed twice using the i variable >> after the for iteration. What happens is that in the last iteration >> the i index is incremented to 16, checked (i<16) then confirmed >> exiting the loop. >> >> 277 for (i=0; i<16; i+=2) { ... >> >> Later there are reading attempts to the pteg last elements, but using >> again the already incremented i (16). >> >> 303 v = be64_to_cpu(pteg[i]); /* pteg[16] */ >> 304 r = be64_to_cpu(pteg[i+1]); /* pteg[17] */ > > Was it some automated tool that came up with this? Yep, cppcheck. As I'm still not engaged to a specific area in the kernel, just trying to help with automated catches. > > There is actually no problem because the accesses outside the loop are > only done if the 'found' variable is true; 'found' is initialized to > false and only ever set to true inside the loop just before a break > statement. Thus there is a correlation between the value of 'i' and > the value of 'found' -- if 'found' is true then we know 'i' is less > than 16. I figured it out after your explanation. > >> I really don't know if the for lace will somehow iterate until i is >> 16, anyway I think that the last readings must be using a defined max >> len/index or another more clear method. > > I think it's perfectly clear to a human programmer, though some tools > (such as gcc) struggle with this kind of correlation between > variables. That's why I asked whether your report was based on the > output from some tool. > >> Eg. >> >> v = be64_to_cpu(pteg[PTEG_LEN - 2]); >> r = be64_to_cpu(pteg[PTEG_LEN - 1]); >> >> Or just. >> >> v = be64_to_cpu(pteg[14]); >> r = be64_to_cpu(pteg[15]); > > Either of those options would cause the code to malfunction. Yep, I understand now that v and r get the found ones. So i is needed. > >> I found in the same file a variable that is not used. >> >> 380 struct kvmppc_vcpu_book3s *vcpu_book3s; >> ... >> 387 vcpu_book3s = to_book3s(vcpu); > > True. It could be removed. I'll make a patch for that. > >> A question, the kvmppc_mmu_book3s_64_init function is accessed by >> unconventional way? Because I have not found any calling to it. > > Try arch/powerpc/kvm/book3s_pr.c line 410: > > kvmppc_mmu_book3s_64_init(vcpu); > > Grep (or git grep) is your friend. Hmm, indeed. > > Paul. Thank you, Paul. If you have some other changes in progress let me know. -- Regards, Geyslan G. Bem hackingbits.com -- 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/