On Mon, Jul 18, 2016 at 09:26:29AM -0400, Vivek Goyal wrote: > On Mon, Jul 18, 2016 at 10:46:04PM +1000, Balbir Singh wrote: > > 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 > > IIUC, sig_enforce bool_enable_only so it can only be enabled. Default > value of it is 0 if CONFIG_MODULE_SIG_FORCE=n. > > IOW, if your kernel forced signature verification, you should not be > able to do sig_enforce=0. If you kernel did not have > CONFIG_MODULE_SIG_FORCE=y, then sig_enforce should be 0 by default anyway > and you are not making it worse using command line. [ CC Matthew Garrett ] I think on top of this there were patches by Matthew Garrett, which disallowed loading of unsigned modules if booted with secureboot on. I think those patches never made upstream though. Vivek > > > > > > > > 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? > > Well, idea was that if you have physical access to machine, then all > bets are off. If BMC can do something which allows running unsigned > code at ring level 0, its a problem I think from secureboot model of > security. > > > ? > > > >? > > > >? > > > > 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 > > If changing command line allows execution of unsigned code at ring level > 0, then it is a problem. Otherwise we are talking of security issues which > are not covered by secureboot model. > > Vivek
