kexec_load(2) bypasses signature verification

From: "Theodore Ts'o" <tytso@mit.edu>
To: Eric Biederman <ebiederm@xmission.com>, dhowells@redhat.com
Cc: kexec@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: kexec_load(2) bypasses signature verification
Date: Sun, 14 Jun 2015 23:50:51 -0400	[thread overview]
Message-ID: <20150615035051.GA2634@thunk.org> (raw)

>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.

(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?

						- Ted