Russell King - ARM Linux <linux at armlinux.org.uk> writes: > On Wed, Jul 13, 2016 at 02:59:51PM +1000, Stewart Smith wrote: >> Russell King - ARM Linux <linux at armlinux.org.uk> writes: >> > On Tue, Jul 12, 2016 at 10:58:05PM +0200, Petr Tesarik wrote: >> >> I'm not an expert on DTB, so I can't provide an example of code >> >> execution, but you have already mentioned the /chosen/linux,stdout-path >> >> property. If an attacker redirects the bootloader to an insecure >> >> console, they may get access to the system that would otherwise be >> >> impossible. >> > >> > I fail to see how kexec connects with the boot loader - the DTB image >> > that's being talked about is one which is passed from the currently >> > running kernel to the to-be-kexec'd kernel. For ARM (and I suspect >> > also ARM64) that's a direct call chain which doesn't involve any >> > boot loader or firmware, and certainly none that would involve the >> > passed DTB image. >> >> For OpenPOWER machines, kexec is the bootloader. Our bootloader is a >> linux kernel and initramfs with a UI (petitboot) - this means we never >> have to write a device driver twice: write a kernel one and you're done >> (for booting from the device and using it in your OS). > > I think you misunderstood my point. > > On ARM, we do not go: > > kernel (kexec'd from) -> boot loader -> kernel (kexec'd to) > > but we go: > > kernel (kexec'd from) -> kernel (kexec'd to) > > There's no intermediate step involving any bootloader. > > Hence, my point is that the dtb loaded by kexec is _only_ used by the > kernel which is being kexec'd to, not by the bootloader, nor indeed > the kernel which it is loaded into. > > Moreover, if you read the bit that I quoted (which is what I was > replying to), you'll notice that it is talking about the DTB loaded > by kexec somehow causing the _bootloader_ to be redirected to an > alternative console. This point is wholely false on ARM. Ahh.. I missed the bootloader bit there. In which case, we're the same on OpenPOWER, there is no intermediate bootloader - in our case we have linux (with kexec) taking on what uboot or grub is typically used for on other platforms. -- Stewart Smith OPAL Architect, IBM.
