From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (ozlabs.org [IPv6:2401:3900:2:1::2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 09DDC1A080F for ; Mon, 30 Nov 2015 08:33:44 +1100 (AEDT) Date: Mon, 30 Nov 2015 08:33:00 +1100 From: Paul Mackerras To: Geyslan =?iso-8859-1?Q?Greg=F3rio?= Bem 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 Subject: Re: [RFC] kvm - possible out of bounds Message-ID: <20151129213300.GA28845@fergus.ozlabs.ibm.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 In-Reply-To: List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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? 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 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. > 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. > 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. Paul.