>>>>> >>>>> 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. > OK.. I checked and you are right, but that is an example and there are other things like security=, thermal.*, nosmep, nosmap that need auditing for safety and might hurt the system security if used. I still think think that assuming you can pass any command line without breaking security is a broken argument. >> >>>>> 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 secure I agree that from what I can see/grep there is nothing that allows unsigned code to run at boot in ring0, but there are implications like the ones I've mentioned above. Attacks are typically built as a chain and every bit might matter. One could turn off features that might lead to the system being attacked at run-time Balbir Singh.
