Re: [PATCH 3/4] ima: add support for KEXEC_ORIG_KERNEL_CHECK

[Seth Forshee <seth.forshee@canonical.com>]

On Fri, Aug 03, 2018 at 10:54:59AM -0400, Mimi Zohar wrote:
> On Fri, 2018-08-03 at 08:11 -0500, Seth Forshee wrote:
> > On Wed, Jul 25, 2018 at 06:31:59PM -0500, Eric Richter wrote:
> > > IMA can verify the signature of kernel images loaded with kexec_file_load,
> > > but can not verify images loaded with the regular kexec_load syscall.
> > > Therefore, the appraisal will automatically fail during kexec_load when an
> > > appraise policy rule is set for func=KEXEC_KERNEL_CHECK. This can be used
> > > to effectively disable the kexec_load syscall, while still allowing the
> > > kexec_file_load to operate so long as the target kernel image is signed.
> > > 
> > > However, this conflicts with CONFIG_KEXEC_VERIFY_SIG. If that option is
> > > enabled and there is an appraise rule set, then the target kernel would
> > > have to be verifiable by both IMA and the architecture specific kernel
> > > verification procedure.
> > > 
> > > This patch adds a new func= for IMA appraisal specifically for the original
> > > kexec_load syscall. Therefore, the kexec_load syscall can be effectively
> > > disabled via IMA policy, leaving the kexec_file_load syscall able to do its
> > > own signature verification, and not require it to be signed via IMA. To
> > > retain compatibility, the existing func=KEXEC_KERNEL_CHECK flag is
> > > unchanged, and thus enables appraisal for both kexec syscalls.
> > 
> > This seems like a roundabout way to disallow the kexec_load syscall.
> > Wouldn't it make more sense to simply disallow kexec_load any time
> > CONFIG_KEXEC_VERIFY_SIG is enabled, since it effectively renders that
> > option impotent? Or has that idea already been rejected?
> 
> Agreed!  We can modify the "case LOADING_KEXEC_IMAGE" in
> ima_load_data() to prevent the kexec_load based on
> CONFIG_KEXEC_VERIFY_SIG.
> 
> The architecture specific policy would only include the IMA appraise
> rule if CONFIG_KEXEC_VERIFY_SIG was not defined.

After looking at this some more I'm having second thoughts about my
suggestion. As a distro we produce a kernel that needs to be flexible
enough for a variety of scenarios, and if we completely close off the
ability to load an unsigned kernel for kexec that's almost certainly
going to end up breaking some use cases.

So I think it is necessary to make this a run-time decision rather than
a compile-time decision. The patch as provided does this based on
whether or not the kernel was booted under secure boot. That might be
reasonable, though I still find this mechanism kind of awkward. It seems
like ideally there would instead be some logic that would accept the
image if the KEXEC_VERIFY_SIG verification had passed, and otherwise
require IMA signature verification.

Thanks,
Seth