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

Mimi Zohar
2018-05-03T21:58:07Z

On Thu, 2018-05-03 at 16:38 -0500, Eric W. Biederman wrote:
> Mimi Zohar <zohar@linux.vnet.ibm.com> writes:
>
> > [Cc'ing Kees and kernel-hardening]
> >
> > On Thu, 2018-05-03 at 15:13 -0500, Eric W. Biederman wrote:
> >> Mimi Zohar <zohar@linux.vnet.ibm.com> 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.  The concern is not for those building their own kernels,
> > but for those using stock kernels.
> >
> > 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.  Currently,
> > 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.  Without a call in kexec_load that isn't possible.

>
> >> 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.

Mimi

>
> >> Given that you are not trusting userspace this changeset also probably
> >> needs to have the kernel-hardening list cc'd. Because the only possible
> >> justification I can imagine for something like this is kernel-hardening.
> >
> > Sure, Cc'ing linux-hardening and Kees.
> >
> > Mimi
>