Re: [RFC 0/3] extend kexec_file_load system call

[Russell King - ARM Linux]

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.
> 
> 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 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.

-- 
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