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  12   <h1>The rEFInd Boot Manager:<br />Managing Secure Boot</h1>
  13
  14   <p class="subhead">by Roderick W. Smith, <a
  15 href="mailto:rodsmith@rodsbooks.com">rodsmith@rodsbooks.com</a></p>
  16
  17 <p>Originally written: 11/13/2012; last Web page update:
  18 12/8/2012, referencing rEFInd 0.5.0.1</p>
  19
  20
  21 <p>I'm a technical writer and consultant specializing in Linux technologies. This Web page is provided free of charge and with no annoying outside ads; however, I did take time to prepare it, and Web hosting does cost money. If you find this Web page useful, please consider making a small donation to help keep this site up and running. Thanks!</p>
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  86
  87 <hr />
  88
  89 <p>This page is part of the documentation for the rEFInd boot manager. If a Web search has brought you here, you may want to start at the <a href="index.html">main page.</a></p>
  90
  91 <hr />
  92
  93 <p class="sidebar"><b>Note:</b> My <a href="http://www.rodsbooks.com/efi-bootloaders/">Managing EFI Boot Loaders for Linux</a> Web page includes a much more detailed description of Secure Boot in its <a href="http://www.rodsbooks.com/efi-bootloaders/secureboot.html">Dealing with Secure Boot</a> sub-page. You should consult this page if you want to disable Secure Boot, generate your own keys, or perform other such tasks.</p>
  94
  95 <p>If you're using a computer that supports Secure Boot, you may run into extra complications. This feature is intended to make it difficult for malware to insert itself early into the computer's boot process. Unfortunately, it also complicates multi-boot configurations such as those that rEFInd is intended to manage. This page describes some <a href="#basic">secure boot basics</a> and two specific aspects of rEFInd and its interactions with Secure Boot: <a href="#installation">installation issues</a> and <a href="#mok">MOK management.</a> It concludes with a look at <a href="#caveats">known bugs and limitations</a> in rEFInd's Secure Boot features.</p>
  96
  97 <a name="basic">
  98 <h2>Basic Issues</h2>
  99 </a>
 100
 101 <p class="sidebar"><b>Note:</b> You don't <i>have to</i> use Secure Boot. If you don't want it, you can <a href="http://www.rodsbooks.com/efi-bootloaders/secureboot.html#disable">disable it,</a> at least on <i>x</i>86-64 PCs. If an ARM-based computer ships with Windows 8, this isn't an option for it. Unfortunately, the shim software described on this page currently supports only x86-64, not x86 or ARM.</p>
 102
 103 <p>Through 2012, it became obvious that Secure Boot would be a feature that was controlled, to a large extent, by Microsoft. This is because Microsoft requires that non-server computers that display Windows 8 logos ship with Secure Boot enabled. As a practical matter, this also means that such computers ship with Microsoft's keys in their firmware. In the absence of an industry-standard body to manage the signing of Secure Boot keys, this means that Microsoft's key is the only one that's more-or-less guaranteed to be installed on the computer, thus blocking the ability to boot any OS that lacks a boot path through Microsoft's signing key.</p>
 104
 105 <p>Fortunately, Microsoft will sign third-party binaries with their key. A payment of $99 to Verisign enables a software distributor to sign as many binaries as desired. Red Hat (Fedora), Novell (SUSE), and Canonical (Ubuntu) have all announced plans to take advantage of this system. Unfortunately, using a third-party signing service is an awkward solution for open source software. In fact, for this very reason Red Hat has developed a program that it calls <i>shim</i> that essentially shifts the Secure Boot "train" from Microsoft's proprietary "track" to one that's more friendly to open source authors. Shim is signed by Microsoft and redirects the boot process to another boot loader that can be signed with keys that the distribution maintains and that are built into shim. Fedora 18 is expected to use this system. SUSE has announced that it will use the same system, as does Ubuntu with version 12.10 and later. SUSE has contributed to the shim approach by providing expansions to shim that support a set of keys that users can maintain themselves. These keys are known as Machine Owner Keys (MOKs), and managing them is described later, in <a href="#mok">Managing MOKs.</a> To reiterate, then, there are potentially three ways to sign a binary that will get it launched on a system that uses shim:</p>
 106
 107 <ul>
 108
 109 <li><b>Secure Boot keys</b>&mdash;These keys are managed by the EFI firmware. In a default configuration, Microsoft is the only party that's more-or-less guaranteed to be able to sign boot loaders with these keys; however, it's possible to <a href="http://www.rodsbooks.com/efi-bootloaders/secureboot.html#add_keys">replace Microsoft's keys with your own,</a> in order to take full control of Secure Boot on your computer. The trouble is that this process is tedious and varies in details from one computer to another.</li>
 110
 111 <li><b>Shim's built-in keys</b>&mdash;It's possible, but not necessary, to compile shim with a built-in public key. Its private counterpart can then be used to sign binaries. In practice, this key type is limited in utility; it's likely to be used by distribution maintainers to sign their own version of GRUB and the Linux kernels that it launches, nothing more. On the plus side, shim's keys require little or no maintenance by users. One potential complication is that if you swap out one shim binary for another, its built-in key may change, which means that the replacement shim might no longer launch its follow-on boot loader.</li>
 112
 113 <li><b>MOKs</b>&mdash;Versions 0.2 and later of shim support MOKs, which give you the ability to add your own keys to the computer. If you want to install multiple Linux distributions in Secure Boot mode, MOKs are likely to be helpful. They're vital if you want to launch kernels you compile yourself or use boot managers or boot loaders other than those provided by your distribution.</li>
 114
 115 </ul>
 116
 117 <p>All three key types are the same in form&mdash;shim's built-in keys and MOKs are both generated using the same tools used to generate Secure Boot keys. Unfortunately, the tools used to generate these keys are still rather crude and are rarely installed on Linux systems, which is one of the reasons that rEFInd's installation script doesn't yet support setting up a Secure Boot configuration. Although it's theoretically possible to use rEFInd without signing your own binaries, this is not yet practical, because distributions don't yet provide their own signed binaries or the public MOK files you must have to enroll their keys. With any luck this will change in 2013. At the very least, many distributions will begin supporting Secure Boot in the near future, and with any luck they'll include their public MOKs for use with other distributions' versions of shim.</p>
 118
 119 <p>Because shim and MOK are being supported by several of the major players in the Linux world, I've decided to do the same with rEFInd. Beginning with version 0.5.0, rEFInd can communicate with the shim system to authenticate boot loaders. If a boot loader has been signed by a valid UEFI Secure Boot key, a valid shim key, or a valid MOK key, rEFInd will launch it. rEFInd will also launch unsigned boot loaders or those with invalid signatures <i>if</i> Secure Boot is disabled in or unsupported by the firmware. (If that's your situation, you needn't bother reading this page.)</p>
 120
 121 <p>Version 0.5.0 ships signed with my own keys, and I provide the public version of this key with the rEFInd package. This can help simplify setup, since you needn't generate your own keys to get rEFInd working; however, without public keys for the boot loaders that rEFInd launches, you'll still need to generate keys and sign your boot loaders, as described in the <a href="#mok">Managing Your MOKs</a> section.</p>
 122
 123 <a name="installation">
 124 <h2>Installation Issues</h2>
 125 </a>
 126
 127 <p>A working Secure Boot installation of rEFInd involves at least three programs, and probably four or more, each of which must be installed in a specific way:</p>
 128
 129 <ul>
 130
 131 <li><b>shim</b>&mdash;You can download a version of shim signed with Microsoft's Secure Boot key <a href="http://www.codon.org.uk/~mjg59/shim-signed/">here.</a> This version (created by shim's developer, former Red Hat employee Matthew J. Garrett) includes a shim key that's used by nothing but the <tt>MokManager.efi</tt> program that also ships with the program. Thus, to use this version of shim, you must use MOKs. Ubuntu 12.10 ships with its own shim, but that version doesn't support MOKs and so is useless for launching rEFInd. Future versions of Fedora, SUSE, and probably other distributions will come with their own variants of shim, most of which will no doubt support their own shim keys as well as MOKs. You should install shim just as you would install other EFI boot loaders, as described <a href="http://www.rodsbooks.com/efi-bootloaders/installation.html">here.</a> For use in launching rEFInd, it makes sense to install <tt>shim.efi</tt> in <tt>EFI/refind</tt> on your ESP, although of course this detail is up to you.</li>
 132
 133 <li><b>MokManager</b>&mdash;This program is included with shim 0.2 and later. It presents a crude user interface for managing MOKs, and it's launched by shim if shim can't find its default boot loader (generally <tt>grubx64.efi</tt>) or if that program isn't properly signed. In principle, this program could be signed with a Secure Boot key or a MOK, but the binary in Garrett's shim 0.2 is signed with a shim key, and I expect that versions distributed with most Linux distributions will also be signed by their respective shim keys. This program should reside in the same directory as <tt>shim.efi</tt>, under the name <tt>MokManager.efi</tt>. Although you could theoretically do without MokManager, in practice you'll need it at least temporarily to install the MOK with which rEFInd is signed.</li>
 134
 135 <li><b>rEFInd</b>&mdash;Naturally, you need rEFInd. Because shim is hard-coded to launch a program called <tt>grubx64.efi</tt>, you must install rEFInd using that name and to the same directory in which <tt>shim.efi</tt> resides. In theory, rEFInd could be signed with a Secure Boot key, a shim key, or a MOK; however, because Microsoft won't sign binaries distributed under the GPLv3, I can't distribute a version of rEFInd signed with Microsoft's Secure Boot key; and as I don't have access to the private shim keys used by any distribution, I can't distribute a rEFInd binary signed by them. (If distributions begin including rEFInd in their package sets, though, such distribution-provided binaries could be signed with the distributions' shim keys.) Thus, rEFInd will normally be signed by a MOK. Beginning with version 0.5.0, rEFInd binaries that I provide are signed by me.</li>
 136
 137 <li><b>Your boot loaders and kernels</b>&mdash;Your OS boot loaders, and perhaps your Linux kernels, must be signed. They can be signed with any of the three key types. Indeed, your system may have a mix of all three types&mdash;a Windows 8 boot loader will most likely be signed with Microsoft's Secure Boot key, GRUB and kernels provided by most distributions will be signed with their own shim keys, and if you use your own locally-compiled kernel or a boot loader from an unusual source you may need to sign it with a MOK. Aside from signing, these files can be installed in exactly the same way as if your computer were not using Secure Boot.</li>
 138
 139 </ul>
 140
 141 <p>Because of variables such as which version of shim you're using and whether you're installing a pre-signed version of rEFInd or want to sign it yourself, I can't provide an absolutely complete procedure for installing rEFInd to work with Secure Boot. Broadly speaking, though, the procedure should be something like this:</p>
 142
 143 <ol>
 144
 145 <li>Boot the computer. This can be a challenge in and of itself. You may need to use a Secure Boot&ndash;enabled Linux emergency disc, temporarily disable Secure Boot, or do the work from Windows.</li>
 146
 147 <li><a href="getting.html">Download rEFInd</a> in binary form (the binary zip or CD-R image file). If you download the binary zip file, unzip it; if you get the CD-R image file, burn it to a CD-R and mount it.</li>
 148
 149 <li>Download shim from <a href="http://www.codon.org.uk/~mjg59/shim-signed/">Matthew J. Garrett's download site</a> or from your distribution. (Don't use Ubuntu 12.10's version, though; as noted earlier, it's inadequate for use with rEFInd.)</li>
 150
 151 <li>Copy the <tt>shim.efi</tt> and <tt>MokManager.efi</tt> binaries to the directory you intend to use for rEFInd&mdash;for instance, <tt>EFI/refind</tt> on the ESP.</li>
 152
 153 <li>Follow the installation instructions for rEFInd on the <a href="installing.html">Installing rEFInd</a> page; however, give rEFInd the filename <tt>grubx64.efi</tt> and register <tt>shim.efi</tt> with the EFI by using <tt>efibootmgr</tt> in Linux or <tt>bcdedit</tt> in Windows. This is most cleanly done by following the manual instructions; however, you can use the <tt>install.sh</tt> script if you subsequently rename the files and register <tt>shim.efi</tt> with <tt>efibootmgr</tt>. Be sure that rEFInd (as <tt>grubx64.efi</tt>), <tt>shim.efi</tt>, and <tt>MokManager.efi</tt> all reside in the same directory.</li>
 154
 155 <li>Copy the <tt>refind.cer</tt> file from the rEFInd package to your ESP, ideally to a location with few other files. (The rEFInd installation directory should work fine.)</li>
 156
 157 <li>Reboot. With any luck, you'll see a simple text-mode user interface with a label of <tt>Shim UEFI key management</tt>. This is the MokManager program, which shim launched when rEFInd failed verification because its key is not yet enrolled.</li>
 158
 159 <li>Press your down arrow key and press Enter to select <tt>Enroll key from disk</tt>. The screen will clear and prompt you to select a key, as shown here:</li>
 160
 161     <br /><IMG SRC="MokManager1.png" ALIGN="CENTER" WIDTH="676"
 162     HEIGHT="186" ALT="MokManager's user interface is crude but effective."
 163     BORDER=2> <br />
 164
 165 <li>Each of the lines with a long awkward string represents a disk partition. Select one and you'll see a list of files. Continue selecting subdirectories until you find the <tt>refind.cer</tt> file you copied to the ESP earlier.</li>
 166
 167 <li>Select <tt>refind.cer</tt>. You can type <tt class="userinput">1</tt> to view the certificate's details if you like, or skip that and type <tt class="userinput">0</tt> to enroll the key.</li>
 168
 169 <li>Back out of any directories you entered and return to the MokManager main menu.</li>
 170
 171 <li>Select <tt>Continue boot</tt> at the main menu.</li>
 172
 173 </ol>
 174
 175 <p>At this point the computer may boot into its default OS, reboot, or perhaps even hang. When you reboot it, though, rEFInd should start up in Secure Boot mode. (You can verify this by selecting the <i>About rEFInd</i> tool in the main menu. Check the <i>Platform</i> item in the resulting screen; it should verify that Secure Boot is active.) You should now be able to launch any boot loader signed with a key recognized by the firmware or by shim (including any MOKs you've enrolled). If you want to manage keys in the future, rEFInd displays a new icon in the second (tools) row you can use to launch MokManager. (This icon appears by default if MokManager is installed, but if you edit <tt>showtools</tt> in <tt>refind.conf</tt>, you must be sure to include <tt>mok_tool</tt> as an option in order to gain access to it.)</p>
 176
 177 <p>If you're using Ubuntu 12.10, you can't use its version of shim, but you can replace it with Garrett's shim. The problem is that Ubuntu's GRUB and kernel will then be signed by an unknown key. Unfortunately, I haven't found a suitable public key file on Ubuntu's distribution medium, so you may need to sign GRUB and/or your kernels with your own MOK. In principle, you should be able to use shim 0.2 or later from future distributions that include it; but you must be sure that whatever you use supports MokManager.</p>
 178
 179 <a name="mok">
 180 <h2>Managing Your MOKs</h2>
 181 </a>
 182
 183 <p>The preceding instructions provided the basics of getting rEFInd up and running, including using MokManager to enroll a MOK on your computer. If you need to sign binaries, though, you'll have to use additional tools. The OpenSSL package provides the cryptographic tools necessary, but actually signing EFI binaries requires additional software. Two packages for this are available: <tt>sbsigntool</tt> and <tt>pesign</tt>. Both are available in binary form from <a href="https://build.opensuse.org/project/show?project=home%3Ajejb1%3AUEFI">this OpenSUSE Build Service (OBS)</a> repository. The following procedure uses <tt>sbsigntool</tt>. To sign your own binaries, follow these steps:</p>
 184
 185 <ol>
 186
 187 <li>If it's not already installed, install OpenSSL on your computer. (It normally comes in a package called <tt>openssl</tt>.</li>
 188
 189 <li>Type the following two commands to generate your public and private keys:
 190
 191 <pre class="listing">
 192 $ <tt class="userinput">openssl req -new -x509 -newkey rsa:2048 -keyout MOK.key -out MOK.crt \
 193   -nodes -days 3650 -subj "/CN=Your Name/"</tt>
 194 $ <tt class="userinput">openssl x509 -in MOK.crt -out MOK.cer -outform DER</tt>
 195 </pre>
 196
 197 Change <tt>Your Name</tt> to your own name or other identifying characteristics, and adjust the certificate's time span (set via <tt>-days</tt>) as you see fit. If you omit the <tt>-nodes</tt> option, the program will prompt you for a passphrase for added security. Remember this, since you'll need it to sign your binaries. The result is a private key file (<tt>MOK.key</tt>), which is highly sensitive since it's required to sign binaries, and two public keys (<tt>MOK.crt</tt> and <tt>MOK.cer</tt>), which can be used to verify signed binaries' authenticity. The two public key files are equivalent, but are used by different tools&mdash;<tt>sbsigntool</tt> uses <tt>MOK.crt</tt> to sign binaries, but MokManager uses <tt>MOK.cer</tt> to enroll the key.</li>
 198
 199 <li>Copy the three key files to a secure location and adjust permissions such that only you can read <tt>MOK.key</tt>. You'll need these keys to sign future binaries, so don't discard them.</li>
 200
 201 <li>Copy the <tt>MOK.cer</tt> file to your ESP, ideally to a location with few other files. (MokManager's user interface becomes unreliable when browsing directories with lots of files.)</li>
 202
 203 <li>Download and install the <tt>sbsigntool</tt> package. Binary links for various distributions are available from the <a href="https://build.opensuse.org/package/show?package=sbsigntools&project=home%3Ajejb1%3AUEFI">OpenSUSE Build Service</a>, or you can obtain the source code by typing <tt class="userinput">git clone git://kernel.ubuntu.com/jk/sbsigntool</tt>.</li>
 204
 205 <li>Sign your binary by typing <tt class="userinput">sbsign --key MOK.key --cert MOK.crt --output <tt class="variable">binary-signed.efi binary.efi</tt></tt>, adjusting the paths to the keys and the binary names.</li>
 206
 207 <li>Copy your signed binary to a suitable location on the ESP for rEFInd to locate it. Be sure to include any support files that it needs, too.</li>
 208
 209 <li>Check your <tt>refind.conf</tt> file to ensure that the <tt>showtools</tt> option is either commented out or includes <tt>mok_tool</tt> among its options.</li>
 210
 211 <li>Reboot. You can try launching the boot loader you just installed, but chances are it will generate an <tt>Access Denied</tt> message. For it to work, you must launch MokManager using the tool that rEFInd presents on its second row. You can then enroll your <tt>MOK.cer</tt> key just as you enrolled the <tt>refind.cer</tt> key.</li>
 212
 213 </ol>
 214
 215 <p>At this point you should be able to launch the binaries you've signed. Unfortunately, there can still be problems at this point....</p>
 216
 217 <a name="caveats">
 218 <h2>Secure Boot Caveats</h2>
 219 </a>
 220
 221 <p>rEFInd's Secure Boot support is brand-new with version 0.5.0 of the program. Unfortunately, rEFInd, like shim, must essentially bypass UEFI security features, and must simultaneously not create security problems, in order to work. Unfortunately, the procedures that rEFInd uses to do this (which were lifted straight from shim) play "fast and loose" with the UEFI rules. This fact creates a number of limitations, which include (but are almost certainly not limited to) the following:</p>
 222
 223 <ul>
 224
 225 <li>rEFInd can launch <i>one</i> shim/MOK-signed driver, no more. If you
 226     try to launch two drivers, rEFInd throws up an <tt>Access Denied</tt>
 227     error for the second driver.</li>
 228
 229 <li>ELILO can't find the directory from which it was launched when launched
 230     from rEFInd in Secure Boot mode. This means that you must pass the
 231     <tt>-C <tt class="variable">/path/to/binary/</tt>elilo.conf</tt> option
 232     to ELILO. rEFInd does this automatically for the default ELILO option,
 233     but you should bear the need in mind if you edit that option or use the
 234     secondary boot options. Because of the same problem, you must specify
 235     the complete path to your kernel and initial RAM disk file in
 236     <tt>elilo.conf</tt>. Be sure to specify these paths using either
 237     forward slashes (<tt>/</tt>) or doubled-up backslashes (<tt>\\</tt>).
 238     It's possible that some other boot loaders will suffer from the same
 239     problem.</li>
 240
 241 <li>Signing the Windows boot loader with a MOK won't work; it hangs,
 242     probably for reasons similar to the ones that cause ELILO to fail to
 243     find its home directory. Fortunately, the Windows 8 boot loader should
 244     work because it should be verified and launched via EFI calls rather
 245     than via the new shim-derived code. (I lack a Windows 8 installation
 246     for testing, though.) This limitation could affect you if you want to
 247     boot Windows 7 with Secure Boot active, though.</li>
 248
 249 <li>Under certain circumstances, the time required to launch a boot loader
 250     can increase. This is unlikely to be noticeable for the average small
 251     boot loader, but could be significant for larger boot loaders on slow
 252     filesystems, such as Linux kernels on ext2fs, ext3fs, or ReiserFS
 253     partitions.</li>
 254
 255 <li>Secure Boot mode doesn't work on <i>x</i>86 (IA32) or ARM systems, just
 256     on <i>x</i>86-64 (AMD64) computers. This is largely because shim has
 257     the same limitations.</li>
 258
 259 </ul>
 260
 261 <p>My focus in testing rEFInd's Secure Boot capabilities has been on getting Linux kernels with EFI stub loaders to launch correctly. I've done some minimal testing with GRUB 2, though. I've also tested some self-signed binaries, such as an EFI shell and MokManager. (The EFI shell launches, but will not itself launch anything that's not been signed with a UEFI Secure Boot key. This of course limits its utility.)</p>
 262
 263 <p>At the moment, I consider rEFInd's shim/MOK support to be of alpha quality. I'm releasing it in this state in the hope of getting feedback from adventurous early adopters. I expect to improve the installation procedure, and with any luck fix some of the known bugs, in the next couple of versions. Some of the usability improvements are dependent upon MOK-capable versions of shim being released with major distributions; such versions of shim, with kernels signed with the key that matches the one built into shim, will greatly reduce the need for users to sign boot loaders.</p>
 264
 265 <hr />
 266
 267 <p>copyright &copy; 2012 by Roderick W. Smith</p>
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