kexec_load(2) bypasses signature verification
jwboyer at fedoraproject.org
Mon Jun 15 05:14:19 PDT 2015
On Sun, Jun 14, 2015 at 11:50 PM, Theodore Ts'o <tytso at 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.
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