[kernel-hardening] [PATCHv2 7/7] docs: add dmesg encryption doc

[Dan Aloni <dan@kernelim.com>]

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dan Aloni <dan@kernelim.com>
---
 Documentation/admin-guide/dmesg-encryption.rst | 118 +++++++++++++++++++++++++
 Documentation/admin-guide/index.rst            |   1 +
 2 files changed, 119 insertions(+)
 create mode 100644 Documentation/admin-guide/dmesg-encryption.rst

diff --git a/Documentation/admin-guide/dmesg-encryption.rst b/Documentation/admin-guide/dmesg-encryption.rst
new file mode 100644
index 000000000000..5aedb8db3a7c
--- /dev/null
+++ b/Documentation/admin-guide/dmesg-encryption.rst
@@ -0,0 +1,118 @@
+Kernel message encryption
+-------------------------
+
+.. CONTENTS
+..
+.. - Overview
+.. - Reason for encrypting dmesg
+.. - Compile time and run time switches
+.. - Limitations
+.. - Decrypting dmesg
+
+
+========
+Overview
+========
+
+Similar to the module signing facility, it is also possible to have the kernel
+perform public key encryption of the kernel messages that are being generated
+by printk calls.
+
+The encryption can be performed for one of the trusted public keys in the
+kernel keyring, and by default will be performed against the kernel's module
+signing key.
+
+To prevent a run-time dependency inside printk itself, the encryption takes
+place upon trying to read ``/dev/kmsg`` which is the mechanism currently used
+by ``systemd`` to read kernel messages, and is also used by ``dmesg``
+invocations.
+
+The first line being read by a ``dmesg`` opener will be an artificial line
+containing an encrypted symmetric encryption session key, in RSA PKCS#1 format.
+The other lines are messages encrypted under an AES-128-GCM scheme. All binary
+ciphertext is base64-encoded, so that the ciphertext solely comprises of
+printable characters.
+
+===========
+Limitations
+===========
+
+There are various limitations one need to consider when enabling dmesg
+encryption:
+
+  * The metadata of kernel messages is not part of the encryption (timestamp,
+    log facility, log severity).
+
+  * The seldom accompanying dictionary is also not part of the encryption.
+
+  * Any output to any system console, happening when printk() itself is
+    executing, is also not encrypted. A potential attacker can load up
+    ``netconsole`` and have kernel messages being sent as plaintext to other
+    machines. Hopefully, on embedded devices, all system consoles are under
+    strict control of the developers.
+
+  * The syslog system call is barred from reading kmsg. Its present users are
+    few, as the system call's interface is mostly a fallback to an inaccessible
+    ``/dev/kmsg``. This is only an implementation limitation and that may be
+    addressed.
+
+  * kmsg buffers will still be saved as plaintext inside kdumps. The assumption
+    is that having an access to read a kdump is equivalent to full kernel
+    access anyway.
+
+===========================
+Reason for encryption dmesg
+===========================
+
+For years, dmesg has contained data which could be utilized by vulnerability
+exploiters, allowing for privilege escalations. Developers may leave key data
+such as pointers, indication of driver bugs, and more.
+
+The feature is mostly aimed for device manufacturers who are not keen on
+revealing the full details of kernel execution, bugs, and crashes to their
+users, but only to their developers, so that local programs running on the
+devices cannot use the data for 'rooting' and executing exploits.
+
+==================================
+Compile time and run time switches
+==================================
+
+In build time, this feature is controlled via the ``CONFIG_KMSG_ENCRYPTION``
+configuration variable.
+
+In run time, it can be turned off by providing `kmsg_encrypt=0` as a boot time
+parameter.
+
+================
+Decrypting dmesg
+================
+
+A supplied program in the kernel tree named ``dmesg-decipher`` uses the OpenSSL
+library along with the paired private key of the encryption in order to
+decipher an encrypted dmesg.
+
+An innocuous dmesg invocation will appear as such (with the ciphertexts
+shortened here for the brevity of this document)::
+
+    [    0.000000] K:Zzgt0ovlRvwH....fQgbQ2tdjOzgYFwrzHU00XO4=
+    [    0.000000] M:ogoKk3kCb6q5....1z8BVLr903/w==,16,12
+    [    0.000000] M:CcxUnMRIHrjD....o+c1Zes=,16,12
+    ....
+
+The artificial ``K:`` message is generated per opening of ``/dev/kmsg``. It
+contains the encrypted session key. The encrypted dmesg lines follows it
+(prefix ``M:``).
+
+Provided with the private key, deciphering a dmesg output should be a
+straightforward process.
+
+For example, one can save an encrypted dmesg to ``dmesg.enc`` in one machine,
+then transfer it to another machine which contains access to the PEM with the
+decrypting private key, and use the the following command::
+
+    cat dmesg.enc | ./tools/kmsg/dmesg-decipher certs/signing_key.pem
+
+    [    0.000000] Linux version 4.15.0-rc5+ (dan@jupiter) (gcc version 7.2.1 20170915 (Red Hat 7.2.1-2) (GCC)) #109 SMP Sat Dec 30 18:32:25 IST 2017
+    [    0.000000] Command line: BOOT_IMAGE=/vmlinuz-4.15.0-rc5-dan+ root=UUID=f48b37ec-fcb8-4689-b12e-58703db3cb21 ro rhgb quiet LANG=en_US.UTF-8
+    [    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
+    ...
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 5bb9161dbe6a..3b0cd49c75d4 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -63,6 +63,7 @@ configure specific aspects of kernel behavior to your liking.
    pm/index
    thunderbolt
    LSM/index
+   dmesg-encryption
 
 .. only::  subproject and html
 
-- 
2.14.3