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>

3 files changed, 8 insertions, 3 deletions

diff --git a/kernel/module.c b/kernel/module.c
index efa1e6031950..c1acb0c5b637 100644
--- a/kernel/module.c
+++ b/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
 
diff --git a/kernel/panic.c b/kernel/panic.c
index 6d6300375090..0e25fe10871e 100644
--- a/kernel/panic.c
+++ b/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().
  */
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 031cc5655a51..3cdbed1fbdc7 100644
--- a/kernel/tracepoint.c
+++ b/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;