Fix: module signature vs tracepoints: add new TAINT_UNSIGNED_MODULE

Users have reported being unable to trace non-signed modules loaded
within a kernel supporting module signature.

This is caused by tracepoint.c:tracepoint_module_coming() refusing to
take into account tracepoints sitting within force-loaded modules
(TAINT_FORCED_MODULE). The reason for this check, in the first place, is
that a force-loaded module may have a struct module incompatible with
the layout expected by the kernel, and can thus cause a kernel crash
upon forced load of that module on a kernel with CONFIG_TRACEPOINTS=y.

Tracepoints, however, specifically accept TAINT_OOT_MODULE and
TAINT_CRAP, since those modules do not lead to the "very likely system
crash" issue cited above for force-loaded modules.

With kernels having CONFIG_MODULE_SIG=y (signed modules), a non-signed
module is tainted re-using the TAINT_FORCED_MODULE taint flag.
Unfortunately, this means that Tracepoints treat that module as a
force-loaded module, and thus silently refuse to consider any tracepoint
within this module.

Since an unsigned module does not fit within the "very likely system
crash" category of tainting, add a new TAINT_UNSIGNED_MODULE taint flag
to specifically address this taint behavior, and accept those modules
within Tracepoints. We use the letter 'X' as a taint flag character for
a module being loaded that doesn't know how to sign its name (proposed
by Steven Rostedt).

Also add the missing 'O' entry to trace event show_module_flags() list
for the sake of completeness.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
NAKed-by: Ingo Molnar <mingo@redhat.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: David Howells <dhowells@redhat.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/ABI/testing/sysfs-module

@@ -49,3 +49,4 @@ Description:	Module taint flags:
 			O - out-of-tree module
 			F - force-loaded module
 			C - staging driver module
+			X - unsigned module

Documentation/module-signing.txt

@@ -53,7 +53,8 @@ This has a number of options available:
 
      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.
+     be marked as being tainted, and the concerned modules will be marked as
+     tainted, shown with the character 'X'.
 
      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

Documentation/oops-tracing.txt

@@ -265,6 +265,9 @@ characters, each representing a particular tainted value.
 
  13: 'O' if an externally-built ("out-of-tree") module has been loaded.
 
+ 14: 'X' if an unsigned module has been loaded in a kernel supporting
+     module signature.
+
 The primary reason for the 'Tainted: ' string is to tell kernel
 debuggers if this is a clean kernel or if anything unusual has
 occurred.  Tainting is permanent: even if an offending module is

Documentation/sysctl/kernel.txt

@@ -792,6 +792,8 @@ can be ORed together:
 1024 - A module from drivers/staging was loaded.
 2048 - The system is working around a severe firmware bug.
 4096 - An out-of-tree module has been loaded.
+8192 - An unsigned module has been loaded in a kernel supporting module
+       signature.
 
 ==============================================================
 

include/linux/kernel.h

@@ -469,6 +469,7 @@ extern enum system_states {
 #define TAINT_CRAP			10
 #define TAINT_FIRMWARE_WORKAROUND	11
 #define TAINT_OOT_MODULE		12
+#define TAINT_UNSIGNED_MODULE		13
 
 extern const char hex_asc[];
 #define hex_asc_lo(x)	hex_asc[((x) & 0x0f)]

include/trace/events/module.h

@@ -22,8 +22,10 @@ struct module;
 
 #define show_module_flags(flags) __print_flags(flags, "",	\
 	{ (1UL << TAINT_PROPRIETARY_MODULE),	"P" },		\
+	{ (1UL << TAINT_OOT_MODULE),		"O" },		\
 	{ (1UL << TAINT_FORCED_MODULE),		"F" },		\
-	{ (1UL << TAINT_CRAP),			"C" })
+	{ (1UL << TAINT_CRAP),			"C" },		\
+	{ (1UL << TAINT_UNSIGNED_MODULE),	"X" })
 
 TRACE_EVENT(module_load,
 

kernel/module.c

@@ -1013,6 +1013,8 @@ static size_t module_flags_taint(struct module *mod, char *buf)
 		buf[l++] = 'F';
 	if (mod->taints & (1 << TAINT_CRAP))
 		buf[l++] = 'C';
+	if (mod->taints & (1 << TAINT_UNSIGNED_MODULE))
+		buf[l++] = 'X';
 	/*
 	 * TAINT_FORCED_RMMOD: could be added.
 	 * TAINT_UNSAFE_SMP, TAINT_MACHINE_CHECK, TAINT_BAD_PAGE don't
@@ -3214,7 +3216,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
 		pr_notice_once("%s: module verification failed: signature "
 			       "and/or  required key missing - tainting "
 			       "kernel\n", mod->name);
-		add_taint_module(mod, TAINT_FORCED_MODULE, LOCKDEP_STILL_OK);
+		add_taint_module(mod, TAINT_UNSIGNED_MODULE, LOCKDEP_STILL_OK);
 	}
 #endif
 

kernel/panic.c

@@ -210,6 +210,7 @@ static const struct tnt tnts[] = {
 	{ TAINT_CRAP,			'C', ' ' },
 	{ TAINT_FIRMWARE_WORKAROUND,	'I', ' ' },
 	{ TAINT_OOT_MODULE,		'O', ' ' },
+	{ TAINT_UNSIGNED_MODULE,	'X', ' ' },
 };
 
 /**
@@ -228,6 +229,7 @@ static const struct tnt tnts[] = {
  *  'C' - modules from drivers/staging are loaded.
  *  'I' - Working around severe firmware bug.
  *  'O' - Out-of-tree module has been loaded.
+ *  'X' - Unsigned module has been loaded.
  *
  *	The string is overwritten by the next call to print_tainted().
  */

kernel/tracepoint.c

@@ -633,7 +633,8 @@ EXPORT_SYMBOL_GPL(tracepoint_iter_reset);
 #ifdef CONFIG_MODULES
 bool trace_module_has_bad_taint(struct module *mod)
 {
-	return mod->taints & ~((1 << TAINT_OOT_MODULE) | (1 << TAINT_CRAP));
+	return mod->taints & ~((1 << TAINT_OOT_MODULE) | (1 << TAINT_CRAP) |
+			       (1 << TAINT_UNSIGNED_MODULE));
 }
 
 static int tracepoint_module_coming(struct module *mod)
@@ -644,7 +645,7 @@ static int tracepoint_module_coming(struct module *mod)
 	/*
 	 * We skip modules that taint the kernel, especially those with different
 	 * module headers (for forced load), to make sure we don't cause a crash.
-	 * Staging and out-of-tree GPL modules are fine.
+	 * Staging, out-of-tree, and unsigned GPL modules are fine.
 	 */
 	if (trace_module_has_bad_taint(mod))
 		return 0;