linux-kernel - Re: Kdump with signed images

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Message-ID: <20121101144304.GA15821@redhat.com>
Date:	Thu, 1 Nov 2012 10:43:04 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	Matthew Garrett <mjg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Khalid Aziz <khalid@...ehiking.org>, kexec@...ts.infradead.org,
	horms@...ge.net.au, Dave Young <dyoung@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Roberto Sassu <roberto.sassu@...ito.it>,
	Kees Cook <keescook@...omium.org>
Subject: Re: Kdump with signed images

On Thu, Nov 01, 2012 at 10:29:19AM -0400, Mimi Zohar wrote:
> 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."

Only those executables which have extended capability
(say CAP_SIGNATURES_VERIFIED) will be able to call kexec_load() syscall.
Only signed executables will get this capability upon signature verification
(using keys in kernel keyring only).

so any xyz executable will not be able to call kexec_load() until and
unless it is signed with keys kernel trusts. This is similar to signed
module verification.

So I think this does satisfy the requirement matthew specified. Isn't it?
Matthew, what do you think?

Thanks
Vivek
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