Re: kexec_load(2) bypasses signature verification

From: Josh Boyer <jwboyer@fedoraproject.org>
To: "Theodore Ts'o" <tytso@mit.edu>,
	Eric Biederman <ebiederm@xmission.com>,
	David Howells <dhowells@redhat.com>,
	kexec <kexec@lists.infradead.org>,
	"Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>
Subject: Re: kexec_load(2) bypasses signature verification
Date: Mon, 15 Jun 2015 08:14:19 -0400	[thread overview]
Message-ID: <CA+5PVA6t_dEQt1M7HkeHy9znFkPd-a7525vtFq-g4a6VKL+gLQ@mail.gmail.com> (raw)
In-Reply-To: <20150615035051.GA2634@thunk.org>

On Sun, Jun 14, 2015 at 11:50 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> From experimentation and from looking at the sources, it appears that
> the signature checking is only done in the kexec_file_load(2) system
> all, and not in the kexec_load(2) system call.  And I understand why
> -- the signature is not sent from userspace to the kernel in the older
> kexec_load(2) system call.
>
> The problem is that if you use an old version of kexec, it will use
> the old kexec_load(2) system call, and even though
> CONFIG_KEXEC_VERIFY_SIG is enabled, kexec_load(2) will happily load an
> unsigned kernel, and then "kexec -e" will happily boot into it.
>
> Correct me if I am wrong, but this appears to be a hole in Secure Boot
> you could drive a Mack Truck through.

Yes, which is why most of the distro vendors carry an out-of-tree
patch that disables the old kexec in an SB setup.  It would be nice if
we could merge said patches.  However, they depend on Matthew's
secure_modules/trusted_kernel/<whatever name that works> patchset
which has gotten little movement since we came up with a tentative
agreement at LPC 2013.

> (I noticed this because Debian is still using a kexec-tools from the
> stone ages, version 2.0.7, and I was wondering **why** I was able to
> kexec boot completely unsigned kernels.)
>
> It would appear to me that if CONFIG_KEXEC_VERIFY_SIG is enabled, the
> old kexec_load(2) system call should be disabled (and a warning be
> placed in the Kconfig help that the user should have at least verision
> 2.X of kexec-tools if they enable this kernel option).
>
> Am I missing something?

Those sound like fine suggestions to me.

josh