Date: Thu, 1 Nov 2012 09:53:56 -0400
From: Vivek Goyal <vgoyal@redhat.com>
To: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Matthew Garrett <mjg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Khalid Aziz <khalid@gonehiking.org>, kexec@lists.infradead.org,
        horms@verge.net.au, Dave Young <dyoung@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        linux kernel mailing list <linux-kernel@vger.kernel.org>,
        Dmitry Kasatkin <dmitry.kasatkin@intel.com>,
        Roberto Sassu <roberto.sassu@polito.it>,
        Kees Cook <keescook@chromium.org>
Subject: Re: Kdump with signed images
Message-ID: <20121101135356.GA15659@redhat.com>
References: <20121024173651.GE1821@redhat.com>
 <1351145401.18115.78.camel@falcor>
 <20121025141048.GD9377@redhat.com>
 <1351190421.18115.92.camel@falcor>
 <20121025185520.GA17995@redhat.com>
 <1351214158.18115.186.camel@falcor>
 <20121026023916.GA16762@srcf.ucam.org>
 <20121026170609.GB24687@redhat.com>
 <1351276649.18115.217.camel@falcor>
 <20121101131003.GA14573@redhat.com>
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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

Thanks
Vivek
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