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

Mimi Zohar
2018-05-04T02:29:48Z

On Thu, 2018-05-03 at 18:03 -0500, Eric W. Biederman wrote:
> Mimi Zohar <zohar@linux.vnet.ibm.com> writes:
>
> > 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.
>
> 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.

Initially I'm hoping that files will simply come signed, providing
file provenance.  Anything else is gravy.

> >> >> 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.

Existing kernels might not support the newer kexec_file_load syscall,
so packages are currently being built to try one syscall and fallback
to using the other one.  In this case, it has nothing to do with
quality control.

Mimi