[PATCH] Make kernel taint on invalid module signatures configurable

[Matthew Garrett <mjg59@google.com>]

The default kernel behaviour is for unsigned or invalidly signed modules
to load without warning. Right now, If CONFIG_MODULE_SIG is enabled the
kernel will be tainted in this case. Distributions may wish to enable
CONFIG_MODULE_SIG in order to permit signature enforcement, but may not
wish to alter the behaviour of the kernel in the situation where
signature enforcement is not required. Add a new kernel configuration
option to allow the default behaviour to be toggled, and add a kernel
parameter in order to allow tainting to be enabled at runtime.

Signed-off-by: Matthew Garrett <mjg59@google.com>
---

Rusty, with luck this makes it clearer what I'm trying to achieve here.

 Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
 Documentation/admin-guide/module-signing.rst    | 13 +++++++++----
 init/Kconfig                                    | 13 ++++++++++++-
 kernel/module.c                                 |  7 ++++++-
 4 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d9c171ce4190..7ea1e9aeb7d8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2291,6 +2291,12 @@
 			Note that if CONFIG_MODULE_SIG_FORCE is set, that
 			is always true, so this option does nothing.
 
+	module.sig_taint
+			[KNL] When CONFIG_MODULE_SIG is set, this means
+			that modules without (valid) signatures will taint the
+			kernel on load. Note that if CONFIG_MODULE_SIG_TAINT is
+			set, that is always true, so this option does nothing.
+
 	module_blacklist=  [KNL] Do not load a comma-separated list of
 			modules.  Useful for debugging problem modules.
 
diff --git a/Documentation/admin-guide/module-signing.rst b/Documentation/admin-guide/module-signing.rst
index 27e59498b487..96709b93c606 100644
--- a/Documentation/admin-guide/module-signing.rst
+++ b/Documentation/admin-guide/module-signing.rst
@@ -52,10 +52,11 @@ This has a number of options available:
      This specifies how the kernel should deal with a module that has a
      signature for which the key is not known or a module that is unsigned.
 
-     If this is off (ie. "permissive"), then modules for which the key is not
-     available and modules that are unsigned are permitted, but the kernel will
-     be marked as being tainted, and the concerned modules will be marked as
-     tainted, shown with the character 'E'.
+     If this is off (ie. "permissive"), then modules for which the key
+     is not available and modules that are unsigned are permitted. If
+     ``CONFIG_MODULE_SIG_TAINT`` is enabled or the sig_taint parameter is
+     set the kernel will be marked as being tainted, and the concerned
+     modules will be marked as tainted, shown with the character 'E'.
 
      If this is on (ie. "restrictive"), only modules that have a valid
      signature that can be verified by a public key in the kernel's possession
@@ -266,6 +267,10 @@ for which it has a public key.   Otherwise, it will also load modules that are
 unsigned.   Any module for which the kernel has a key, but which proves to have
 a signature mismatch will not be permitted to load.
 
+If ``CONFIG_MODULE_SIG_TAINT`` is enabled or module.sig_taint=1 is
+supplied on the kernel command line, the kernel will be tainted if an
+invalidly signed or unsigned module is loaded.
+
 Any module that has an unparseable signature will be rejected.
 
 
diff --git a/init/Kconfig b/init/Kconfig
index 8514b25db21c..ffad18a3e890 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1749,12 +1749,23 @@ config MODULE_SIG
 	  debuginfo strip done by some packagers (such as rpmbuild) and
 	  inclusion into an initramfs that wants the module size reduced.
 
+config MODULE_SIG_TAINT
+	bool "Taint the kernel on unsigned or incorrectly signed module load"
+	default y
+	depends on MODULE_SIG
+	help
+	  Taint the kernel if an unsigned or untrusted kernel module is loaded.
+	  If this option is enabled, the kernel will be tainted on an attempt
+	  to load an unsigned module or signed modules for which we don't have
+	  a key.
+
 config MODULE_SIG_FORCE
 	bool "Require modules to be validly signed"
 	depends on MODULE_SIG
 	help
 	  Reject unsigned modules or signed modules for which we don't have a
-	  key.  Without this, such modules will simply taint the kernel.
+	  key. Without this, such modules will be loaded successfully but will
+	  (if MODULE_SIG_TAINT is set) taint the kernel.
 
 config MODULE_SIG_ALL
 	bool "Automatically sign all modules"
diff --git a/kernel/module.c b/kernel/module.c
index 40f983cbea81..70c2edc5693c 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -278,6 +278,11 @@ static bool sig_enforce = IS_ENABLED(CONFIG_MODULE_SIG_FORCE);
 module_param(sig_enforce, bool_enable_only, 0644);
 #endif /* !CONFIG_MODULE_SIG_FORCE */
 
+static bool sig_taint = IS_ENABLED(CONFIG_MODULE_SIG_TAINT);
+#ifndef CONFIG_MODULE_SIG_TAINT
+module_param(sig_taint, bool_enable_only, 0644);
+#endif /* !CONFIG_MODULE_SIG_TAINT */
+
 /* Block module loading/unloading? */
 int modules_disabled = 0;
 core_param(nomodule, modules_disabled, bint, 0);
@@ -3660,7 +3665,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
 
 #ifdef CONFIG_MODULE_SIG
 	mod->sig_ok = info->sig_ok;
-	if (!mod->sig_ok) {
+	if (!mod->sig_ok && sig_taint) {
 		pr_notice_once("%s: module verification failed: signature "
 			       "and/or required key missing - tainting "
 			       "kernel\n", mod->name);
-- 
2.14.0.rc1.383.gd1ce394fe2-goog