From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Sat, 11 Oct 2014 20:44:37 +0800 From: Baoquan He To: "H. Peter Anvin" Cc: Vivek Goyal , Kees Cook , linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, x86@kernel.org, ak@linux.intel.com, ebiederm@xmission.com, kexec@lists.infradead.org, whissi@whissi.de, kumagai-atsushi@mxc.nes.nec.co.jp, stable@vger.kernel.org Subject: Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed Message-ID: <20141011124437.GA9847@dhcp-16-116.nay.redhat.com> References: <1412060896-1902-1-git-send-email-bhe@redhat.com> <542B1EC1.8090502@zytor.com> <20141001135237.GA13689@redhat.com> <543553C7.9010906@zytor.com> <20141008192728.GA26517@redhat.com> <20141011031452.GB11560@dhcp-16-116.nay.redhat.com> <543907B5.7060001@zytor.com> <20141011123805.GC11560@dhcp-16-116.nay.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141011123805.GC11560@dhcp-16-116.nay.redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: On 10/11/14 at 08:38pm, Baoquan He wrote: > On 10/11/14 at 03:34am, H. Peter Anvin wrote: > > On 10/10/2014 08:14 PM, Baoquan He wrote: > > >On 10/08/14 at 03:27pm, Vivek Goyal wrote: > > >>On Wed, Oct 08, 2014 at 08:09:59AM -0700, H. Peter Anvin wrote: > > > > > >>>Sorry... this makes no sense. > > >>> > > >>>For x86-64, there is no direct connection between the physical and > > >>>virtual address spaces that the kernel runs in... > > >> > > >>I am sorry I did not understand this one. I thought that initial > > >>relocatable kernel implementaion did not have any direct connection > > >>between virtual and physical address. One could load kernel anywhere > > >>and kernel virtual address will not change and we will just adjust > > >>page tables to map virtual address to right physical address. > > >> > > >>Now handle_relocation() stuff seems to introduce a close coupling > > >>between physical and virtual address. So if kernel shifts by 16MB > > >>in physical address space, then it will shift by equal amount > > >>in virtual address space. So there seems to be a direct connection > > >>between virtual and physical address space in this case. > > > > > >Yeah, it's exactly as Vivek said. > > > > > >Before kaslr was introduced, x86_64 kernel can be put anywhere, and > > >always _text is 0xffffffff81000000. Meanwhile phys_base contains the > > >offset between the compiled addr (namely 0x1000000) and kernel loaded > > >addr. After kaslr implementation was added, as long as kernel loaded > > >addr is different 0x1000000, it will call handle_relocations(). The > > >offset now is added onto each symbols including _text and phys_base > > >becomes 0. > > > > > >It's clearly showing that by checking /proc/kallsyms and value of > > >phys_base. > > > > > > > This really shouldn't have happened this way on x86-64. It has to > > happen this way on i386, but I worry that this may be a serious > > misdesign in kaslr on x86-64. I'm also wondering if there is any > > other fallout of this? Btw, except of this bug, I didn't find other risk of kaslr currently. The code flow is straightforward and clear. > > Yes, this shouldn't happen this way on x86_64. With this patch, those > are fixed as expected. If kernel location is not chosen randomly, we > should not do the relocations handling. If and only if kaslr is enabled > and it relocated the kernel randomly as expected, we do the relocations > handling. > > I think this patch really makes sense and it's simple and won't impact > i386 and other implementations. >