Message-ID: <1351780159.15708.17.camel@falcor>
Subject: Re: Kdump with signed images
From: Mimi Zohar <zohar@linux.vnet.ibm.com>
To: Vivek Goyal <vgoyal@redhat.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>
Date: Thu, 01 Nov 2012 10:29:19 -0400
In-Reply-To: <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> <20121101135356.GA15659@redhat.com>
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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

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