Re: [RFC] Kdump with signed images

Vivek Goyal
2012-10-25T13:54:55Z

On Thu, Oct 25, 2012 at 01:43:59AM -0400, Mimi Zohar wrote:
> On Wed, 2012-10-24 at 13:19 -0400, Vivek Goyal wrote:
> > On Tue, Oct 23, 2012 at 09:44:59AM -0700, Eric W. Biederman wrote:
> > > Matthew Garrett <mjg@redhat.com> writes:
> > >
> > > > On Tue, Oct 23, 2012 at 10:59:20AM -0400, Vivek Goyal wrote:
> > > >
> > > >> But what about creation of a new program which can call kexec_load()
> > > >> and execute an unsigned kernel. Doesn't look like that will be
> > > >> prevented using IMA.
>
> Like the existing kernel modules, kexec_load() is not file descriptor
> based. There isn't an LSM or IMA-appraisal hook here.
>
> > > > Right. Trusting userspace would require a new system call that passes in
> > > > a signature of the userspace binary, and the kernel would then have to
> > > > verify the ELF object in memory in order to ensure that it
> > > > matches the signature. Verifying that the copy on the filesystem is
> > > > unmodified isn't adequate - an attacker could simply have paused the
> > > > process and injected code.
>
> I haven't looked at kexec_load() in detail, but like kernel modules, I
> think the better solution would be to pass a file descriptor, especially
> if you're discussing a new system call. (cc'ing Kees.)

If we decide to go for a new system call, then yes it looks like passing
fd will make sense.

>
> > > Verifying the copy on the filesystem at exec time is perfectly adequate
> > > for gating extra permissions. Certainly that is the model everywhere
> > > else in the signed key chain.
> > >
> > > Where IMA falls short is there is no offline signing capability in IMA
> > > itself. I think EVM may fix that.
>
> I'm not sure what you mean by offline signing capability. IMA-appraisal
> verifies a file's 'security.ima' xattr, which may contain a hash or a
> digital signature. EVM protects a file's metadata, including
> 'security.ima'. 'security.evm' can be either an hmac or a digital
> signature.

By offline we mean that signature generation/signing does not happen on
system in question. It happens say on build time. IIUC, in case of IMA,
security.ima is generated and signed on the system and that means private
key needs to be present on the system?

We wanted something like module signing where modules are signed at
build time and verification happens at load time but no private key
needs to be present on the system.

>
> > [ CCing lkml. I think it is a good idea to open discussion to wider
> > audience. Also CCing IMA/EVM folks ]
>
> thanks!
>
> > Based on reading following wiki page, looks like EVM also does not allow
> > offline signing capability. And EVM is protecting IMA data to protect
> > against offline attack. If we can assume that unisgned kernels can't be
> > booted on the platform, then EVM might not be a strict requirement in
> > this case.
>
> > So as you said, one of the main problem with IMA use to verify /sbin/kexec
> > is that IMA does not provide offline signing capability.
>
> ?

See above.

>
> > >
> > > > Realistically, the only solution here is for
> > > > the kernel to verify that the kernel it's about to boot is signed and
> > > > for it not to take any untrusted executable code from userspace.
>
> >From an IMA, as opposed to an IMA-appraisal, perspective, kexec is
> problematic. IMA maintains a measurement list and extends a PCR with
> the file hash. The measurement list and PCR value are used to attest to
> the integrity of the running system. As the original measurement list
> is lost after kexec, but the PCR value hasn't been reset, the
> measuremnet list and PCR value won't agree.
>
> > > Hogwash. The kernel verifing a signature of /sbin/kexec at exec time is
> > > perfectly reasonable, and realistic. In fact finding a way to trust
> > > small bits of userspace even if root is compromised seems a far superior
> > > model to simply solving the signing problem for /sbin/kexec.
>
> Huh? I don't understand what you're suggesting. Once root has been
> compromised, that's it.
>
> > > Although I do admit some part of the kexec process will need to verify
> > > keys on the images we decide to boot.
>
> Which keys? Isn't the kernel module key builtin to the kernel and
> included in the kernel image signature?

I think Eric is referring to verifying bzImage signature. One can sign
PE/COFF bzImage so IIUC, Eric seems to be suggesting that let /sbin/kexec
verify the integrity/authenticity of bzImage and figure a way out to
verify integrity/authenticity of /sbin/kexec.

That way one does not have to change current logic and most of the logic
of kexec/kdump can still be in user space. Otherwise we shall probably
have to implement a new system call which takes kernel and initrd as
input parameters and kernel does signature verification and prepares
bzImage for loading. This is like re-writing kexec/kdump again with
restricted functionality.

Thanks
Vivek