Re: Kdump with signed images

Vivek Goyal
2012-11-01T13:11:02Z

On Fri, Oct 26, 2012 at 02:37:29PM -0400, Mimi Zohar wrote:
> On Fri, 2012-10-26 at 13:06 -0400, Vivek Goyal wrote:
> > On Fri, Oct 26, 2012 at 03:39:16AM +0100, Matthew Garrett wrote:
> > > On Thu, Oct 25, 2012 at 09:15:58PM -0400, Mimi Zohar wrote:
> > >
> > > > On a running system, the package installer, after verifying the package
> > > > integrity, would install each file with the associated 'security.ima'
> > > > extended attribute. The 'security.evm' digital signature would be
> > > > installed with an HMAC, calculated using a system unique key.
> > >
> > > The idea isn't to prevent /sbin/kexec from being modified after
> > > installation - it's to prevent it from being possible to install a
> > > system that has a modified /sbin/kexec. Leaving any part of this up to
> > > the package installer means that it doesn't solve the problem we're
> > > trying to solve here. It must be impossible for the kernel to launch any
> > > /sbin/kexec that hasn't been signed by a trusted key that's been built
> > > into the kernel, and it must be impossible for anything other than
> > > /sbin/kexec to make the kexec system call.
> >
> > I am kind of lost now so just trying to summarize whatever I have
> > learned so far from this thread.
>
> Thanks for summarizing.
>
> > - So say we can sign /sbin/kexec at build time and distros can do that.
> > - Verify the signature at exec time using kernel keyring and if
> > verification happens successfully, say process gains extra capability.
> > - Use this new capability to determine whether kexec_load() will be
> > successful or not.
> >
> > Even if we can do all this, it still has the issue of being able to
> > stop the process in user space and replace the code at run time
> > and be able to launch unsigned kernel.

Thinking more about it. Can we just keep track whether a process was
ptraced or not and disallow kexec_load() syscall if it was ptraced.
(I am assuming that ptrace is the only way to change process code/data).

So binaries can be signed offline. Signature verification can take place
using kernel keyring at exec() time. And we can keep track of ptraced
processes and disallow calling kexec_load() for such processes. If this
is implementable, this should take care of following requirement raised
by matthew.

************************************************************************
It must be impossible for the kernel to launch any /sbin/kexec that hasn't
been signed by a trusted key that's been built into the kernel, and it
must be impossible for anything other than /sbin/kexec to make the kexec
system call.
*************************************************************************

Thoughts?

Thanks
Vivek