Re: Kdump with signed images

Mimi Zohar
2012-11-01T14:31:57Z

On Thu, 2012-11-01 at 09:53 -0400, Vivek Goyal wrote:
> On Thu, Nov 01, 2012 at 09:10:03AM -0400, Vivek Goyal wrote:
>
> [..]
> > >
> > > > - 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?
>
> Eric responded but my mistake he responded to only me. So I will quickly
> put his idea here.
>
> [start quote]
>
> You can't ptrace a process that has a capability you don't.
>
> That should be enforced in security/commoncap/
>
> [end quote]
>
> This looks like a good idea. Upon verification signed binaries will be
> assigned special capability and then no unsigned binary should be able
> to ptrace signed/verified processes

That's a good generic solution, which I'm all in favor of, but it
doesn't resolve the latter half of Matthrew's requirement "and it must
be impossible for anything other than /sbin/kexec to make the kexec
system call."

thanks,

Mimi