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 at 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.
