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

[Mark Rutland]

On Fri, Jul 15, 2016 at 09:26:10AM -0400, Vivek Goyal wrote:
> On Fri, Jul 15, 2016 at 09:31:02AM +0200, Arnd Bergmann wrote:
> > On Thursday, July 14, 2016 10:44:14 PM CEST Thiago Jung Bauermann wrote:
> > > Am Donnerstag, 14 Juli 2016, 10:29:11 schrieb Arnd Bergmann:
> > 
> > > > 
> > > > Right, but the question remains whether this helps while you allow the
> > > > boot loader to modify the dtb. If an attacker gets in and cannot modify
> > > > the kernel or initid but can modify the DT, a successful attack would
> > > > be a bit harder than having a modified kernel, but you may still need
> > > > to treat the system as compromised.
> > > 
> > > Yes, and the same question also remains regarding the kernel command line.
> > > 
> > > We can have the kernel perform sanity checks on the device tree, just as the 
> > > kernel needs to sanity check the command line.
> > > 
> > > There's the point that was raised about not wanting to increase the attack 
> > > surface, and that's a valid point. But at least in the way Petitboot works 
> > > today, it needs to modify the device tree and pass it to the kernel.
> > > 
> > > One thing that is unavoidable to come from userspace is 
> > > /chosen/linux,stdout-path, because it's Petitboot that knows from which 
> > > console the user is interacting with. The other modification to set 
> > > properties in vga@0 can be done in the kernel.
> > > 
> > > Given that on DTB-based systems /chosen is an important and established way 
> > > to pass information to the operating system being booted, I'd like to 
> > > suggest the following, then:
> > > 
> > > Extend the syscall as shown in this RFC from Takahiro AKASHI, but instead of 
> > > accepting a complete DTB from userspace, the syscall would accept a DTB 
> > > containing only a /chosen node. If the DTB contains any other node, the 
> > > syscall fails with EINVAL. The kernel can then add the properties in /chosen 
> > > to the device tree that it will pass to the next kernel.
> > > 
> > > What do you think?
> > 
> > I think that helps, as it makes the problem space correspond to that
> > of modifying the command line, but I can still come up with countless
> > attacks based on modifications of the /chosen node and/or the command
> > line, in fact it's probably easier than any other node.
> 
> I don't know anything about DTB. So here comes a very basic question. Does
> DTB allow passing an executable blob to kernel or pass the location of
> some unsigned executable code at kernel level. I think from secureboot point of
> view that would be a concern. Being able to trick kernel to execute an
> unsigned code at privileged level.

The DTB itself won't contain executable code.

However, arbitrary bindings could point kernel at such code. For
instance, /chosen/linux,uefi-system-table could point the kernel at a
faked EFI system table, with pointers to malicious code. So
arbitrary modification of /chosen is not safe.

Bindings describe arbitrary system features (devices, firmware
interfaces, etc), so in general they might provide mechanisms to execute
code.

Thanks,
Mark.