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Subject: Re: [PATCH 3/4] ima: add support for KEXEC_ORIG_KERNEL_CHECK
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Seth Forshee <seth.forshee@canonical.com>,
        Eric Richter <erichte@linux.vnet.ibm.com>
Cc:     linux-integrity <linux-integrity@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Justin Forbes <jforbes@redhat.com>
Date:   Fri, 03 Aug 2018 10:54:59 -0400
In-Reply-To: <20180803131129.GS3001@ubuntu-xps13>
References: <20180725233200.761-1-erichte@linux.vnet.ibm.com>
         <20180725233200.761-4-erichte@linux.vnet.ibm.com>
         <20180803131129.GS3001@ubuntu-xps13>
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On Fri, 2018-08-03 at 08:11 -0500, Seth Forshee wrote:
> On Wed, Jul 25, 2018 at 06:31:59PM -0500, Eric Richter wrote:
> > IMA can verify the signature of kernel images loaded with kexec_file_load,
> > but can not verify images loaded with the regular kexec_load syscall.
> > Therefore, the appraisal will automatically fail during kexec_load when an
> > appraise policy rule is set for func=KEXEC_KERNEL_CHECK. This can be used
> > to effectively disable the kexec_load syscall, while still allowing the
> > kexec_file_load to operate so long as the target kernel image is signed.
> > 
> > However, this conflicts with CONFIG_KEXEC_VERIFY_SIG. If that option is
> > enabled and there is an appraise rule set, then the target kernel would
> > have to be verifiable by both IMA and the architecture specific kernel
> > verification procedure.
> > 
> > This patch adds a new func= for IMA appraisal specifically for the original
> > kexec_load syscall. Therefore, the kexec_load syscall can be effectively
> > disabled via IMA policy, leaving the kexec_file_load syscall able to do its
> > own signature verification, and not require it to be signed via IMA. To
> > retain compatibility, the existing func=KEXEC_KERNEL_CHECK flag is
> > unchanged, and thus enables appraisal for both kexec syscalls.
> 
> This seems like a roundabout way to disallow the kexec_load syscall.
> Wouldn't it make more sense to simply disallow kexec_load any time
> CONFIG_KEXEC_VERIFY_SIG is enabled, since it effectively renders that
> option impotent? Or has that idea already been rejected?

Agreed!  We can modify the "case LOADING_KEXEC_IMAGE" in
ima_load_data() to prevent the kexec_load based on
CONFIG_KEXEC_VERIFY_SIG.

The architecture specific policy would only include the IMA appraise
rule if CONFIG_KEXEC_VERIFY_SIG was not defined.

Mimi