Re: [PATCH 0/3] kexec: limit kexec_load syscall

[Eric W. Biederman]

Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> writes:

> On Thu, 2018-05-03 at 16:38 -0500, Eric W. Biederman wrote:
>> Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> writes:
>> 
>> > [Cc'ing Kees and kernel-hardening]
>> >
>> > On Thu, 2018-05-03 at 15:13 -0500, Eric W. Biederman wrote:
>> >> Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> writes:
>> >> 
>> >> > In environments that require the kexec kernel image to be signed, prevent
>> >> > using the kexec_load syscall.  In order for LSMs and IMA to differentiate
>> >> > between kexec_load and kexec_file_load syscalls, this patch set adds a
>> >> > call to security_kernel_read_file() in kexec_load_check().
>> >> 
>> >> Having thought about it some more this justification for these changes
>> >> does not work.  The functionality of kexec_load is already root-only.
>> >> So in environments that require the kernel image to be signed just don't
>> >> use kexec_load.  Possibly even compile kexec_load out to save space
>> >> because you will never need it.  You don't need a new security hook to
>> >> do any of that.  Userspace is a very fine mechanism for being the
>> >> instrument of policy.
>> >
>> > True, for those building their own kernel, they can disable the old
>> > syscalls. ÂThe concern is not for those building their own kernels,
>> > but for those using stock kernels. Â
>> >
>> > By adding an LSM hook here in the kexec_load syscall, as opposed to an
>> > IMA specific hook, other LSMs can piggy back on top of it. ÂCurrently,
>> > both load_pin and SELinux are gating the kernel module syscalls based
>> > on security_kernel_read_file.
>> >
>> > If there was a similar option for the kernel image, I'm pretty sure
>> > other LSMs would use it.
>> >
>> > From an IMA perspective, there needs to be some method for only
>> > allowing signed code to be loaded, executed, etc. - kernel modules,
>> > kernel image/initramfs, firmware, policies.
>> 
>> What is the IMA perspective.  Why can't IMA trust appropriately
>> authorized userspace?
>
> Suppose a system owner wants to define a system wide policy that
> requires all code be signed - kernel modules, firmware, kexec image &
> initramfs, executables, mmapped files, etc - without having to rebuild
> the kernel. ÂWithout a call in kexec_load that isn't possible.

Of course it is.  You just make it a requirement that before an
executable will be signed it will be audited to see that it doesn't
call sys_kexec_load.  Signing presumably means something.  So it should
not be hard to enforce a policy like that on a specialty system call
that most applications will never call.

>> >> If you don't trust userspace that needs to be spelled out very clearly.
>> >> You need to talk about what your threat models are.
>> >> 
>> >> If the only justification is so that that we can't boot windows if
>> >> someone hacks into userspace it has my nack because that is another kind
>> >> of complete non-sense.
>> >
>> > The usecase is the ability to gate the kexec_load usage in stock
>> > kernels.
>> 
>> But kexec_load is already gated.  It requires CAP_SYS_BOOT.
>
> It isn't a matter of kexec_load already being gated, but of wanting a
> single place for defining a system wide policy, as described above.

Signing is only a tool to enforce a policy.  Signing by itself is not a
policy.  Enforcing any quality controls in the signed executables should
trivially prevent kexec_load from being used.

Eric