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Subject: Re: [PATCH 0/3] kexec: limit kexec_load syscall
From:   Mimi Zohar <zohar@linux.vnet.ibm.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>
Cc:     David Howells <dhowells@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com
Date:   Thu, 03 May 2018 17:31:15 -0400
In-Reply-To: <87r2mso5up.fsf@xmission.com>
References: <1523572911-16363-1-git-send-email-zohar@linux.vnet.ibm.com>
         <87r2mso5up.fsf@xmission.com>
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[Cc'ing Kees and kernel-hardening]

On Thu, 2018-05-03 at 15:13 -0500, Eric W. Biederman wrote:
> Mimi Zohar <zohar@linux.vnet.ibm.com> writes:
> 
> > In environments that require the kexec kernel image to be signed, prevent
> > using the kexec_load syscall.  In order for LSMs and IMA to differentiate
> > between kexec_load and kexec_file_load syscalls, this patch set adds a
> > call to security_kernel_read_file() in kexec_load_check().
> 
> Having thought about it some more this justification for these changes
> does not work.  The functionality of kexec_load is already root-only.
> So in environments that require the kernel image to be signed just don't
> use kexec_load.  Possibly even compile kexec_load out to save space
> because you will never need it.  You don't need a new security hook to
> do any of that.  Userspace is a very fine mechanism for being the
> instrument of policy.

True, for those building their own kernel, they can disable the old
syscalls.  The concern is not for those building their own kernels,
but for those using stock kernels.  

By adding an LSM hook here in the kexec_load syscall, as opposed to an
IMA specific hook, other LSMs can piggy back on top of it.  Currently,
both load_pin and SELinux are gating the kernel module syscalls based
on security_kernel_read_file.

If there was a similar option for the kernel image, I'm pretty sure
other LSMs would use it.

From an IMA perspective, there needs to be some method for only
allowing signed code to be loaded, executed, etc. - kernel modules,
kernel image/initramfs, firmware, policies.

> If you don't trust userspace that needs to be spelled out very clearly.
> You need to talk about what your threat models are.
> 
> If the only justification is so that that we can't boot windows if
> someone hacks into userspace it has my nack because that is another kind
> of complete non-sense.

The usecase is the ability to gate the kexec_load usage in stock
kernels.

> 
> Given that you are not trusting userspace this changeset also probably
> needs to have the kernel-hardening list cc'd.  Because the only possible
> justification I can imagine for something like this is kernel-hardening.

Sure, Cc'ing linux-hardening and Kees.

Mimi