From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-x22b.google.com (mail-pf0-x22b.google.com [IPv6:2607:f8b0:400e:c00::22b]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3rtNF84yZWzDqM3 for ; Mon, 18 Jul 2016 22:45:32 +1000 (AEST) Received: by mail-pf0-x22b.google.com with SMTP id p64so30293884pfb.1 for ; Mon, 18 Jul 2016 05:45:32 -0700 (PDT) Message-ID: <1468845964.2800.3.camel@gmail.com> Subject: Re: [RFC 0/3] extend kexec_file_load system call From: Balbir Singh To: Vivek Goyal , Russell King - ARM Linux Cc: Stewart Smith , bhe@redhat.com, arnd@arndb.de, Petr Tesarik , linuxppc-dev@lists.ozlabs.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, AKASHI Takahiro , "Eric W. Biederman" , Thiago Jung Bauermann , dyoung@redhat.com, linux-arm-kernel@lists.infradead.org Date: Mon, 18 Jul 2016 22:46:04 +1000 In-Reply-To: <20160713182247.GA25232@redhat.com> References: <87furf7ztv.fsf@x220.int.ebiederm.org> <50662781.Utjsnse3nb@hactar> <20160712225805.0d27fe5d@hananiah.suse.cz> <20160712221804.GV1041@n2100.armlinux.org.uk> <87twfunneg.fsf@linux.vnet.ibm.com> <20160713073657.GX1041@n2100.armlinux.org.uk> <87poqinf9m.fsf@linux.vnet.ibm.com> <20160713082639.GZ1041@n2100.armlinux.org.uk> <20160713130338.GB16900@redhat.com> <20160713174009.GA1041@n2100.armlinux.org.uk> <20160713182247.GA25232@redhat.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, 2016-07-13 at 14:22 -0400, Vivek Goyal wrote: > On Wed, Jul 13, 2016 at 06:40:10PM +0100, Russell King - ARM Linux wrote: > >  > > On Wed, Jul 13, 2016 at 09:03:38AM -0400, Vivek Goyal wrote: > > >  > > > On Wed, Jul 13, 2016 at 09:26:39AM +0100, Russell King - ARM Linux wrote: > > > >  > > > > Indeed - maybe Eric knows better, but I can't see any situation where > > > > the dtb we load via kexec should ever affect "the bootloader", unless > > > > the "kernel" that's being loaded into kexec is "the bootloader". > > > >  > > > > Now, going back to the more fundamental issue raised in my first reply, > > > > about the kernel command line. > > > >  > > > > On x86, I can see that it _is_ possible for userspace to specify a > > > > command line, and the kernel loading the image provides the command > > > > line to the to-be-kexeced kernel with very little checking.  So, if > > > > your kernel is signed, what stops the "insecure userspace" loading > > > > a signed kernel but giving it an insecure rootfs and/or console? > > > It is not kexec specific. I could do this for regular boot too, right? > > >  > > > Command line options are not signed. I thought idea behind secureboot > > > was to execute only trusted code and command line options don't enforce > > > you to execute unsigned code. > > >  You can set module.sig_enforce=0 and open up the system a bit assuming that you can get a module to load with another attack > > > So it sounds like different class of security problems which you are > > > referring to and not necessarily covered by secureboot or signed > > > kernel. > > Let me give you an example. > >  > > You have a secure boot setup, where the firmware/ROM validates the boot > > loader.  Good, the boot loader hasn't been tampered with. > >  > > You interrupt the boot loader and are able to modify the command line > > for the booted kernel. > >  > > The boot loader loads the kernel and verifies the kernel's signature. > > Good, the kernel hasn't been tampered with.  The kernel starts running. > >  > > You've plugged in a USB drive to the device, and specified a partition > > containing a root filesystem that you control to the kernel.  The > > validated kernel finds the USB drive, and mounts it, and executes > > your own binaries on the USB drive. > You will require physical access to the machine to be able to > insert your usb drive. And IIRC, argument was that if attacker has > physical access to machine, all bets are off anyway. > You don't need physical access -- your machine controller BMC can do the magic for you. So its not always physical access, is it?   > >  > >  > > You run a shell on the console.  You now have control of the system, > > and can mount the real rootfs, inspect it, and work out what it does, > > etc. > >  > > At this point, what use was all the validation that the secure boot > > has done?  Absolutely useless. > >  > > If you can change the command line arguments given to the kernel, you > > have no security, no matter how much you verify signatures.  It's > > the illusion of security, nothing more, nothing less. > >  I agree, if you can change command line arguments, all bets are of lesser value Balbir Singh