debug: add notifier chain debugging

during some development we suspected a case where we left something
in a notifier chain that was from a module that was unloaded already...
and that sort of thing is rather hard to track down.

This patch adds a very simple sanity check (which isn't all that
expensive) to make sure the notifier we're about to call is
actually from either the kernel itself of from a still-loaded
module, avoiding a hard-to-chase-down crash.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 26 insertions, 0 deletions

diff --git a/kernel/notifier.c b/kernel/notifier.c
index 823be11584ef..143fdd77dbf7 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -21,6 +21,10 @@ BLOCKING_NOTIFIER_HEAD(reboot_notifier_list);
 static int notifier_chain_register(struct notifier_block **nl,
                 struct notifier_block *n)
 {
+        if (!kernel_text_address((unsigned long)n->notifier_call)) {
+                WARN(1, "Invalid notifier registered!");
+                return 0;
+        }
         while ((*nl) != NULL) {
                 if (n->priority > (*nl)->priority)
                         break;
@@ -34,6 +38,10 @@ static int notifier_chain_register(struct notifier_block **nl,
 static int notifier_chain_cond_register(struct notifier_block **nl,
                 struct notifier_block *n)
 {
+        if (!kernel_text_address((unsigned long)n->notifier_call)) {
+                WARN(1, "Invalid notifier registered!");
+                return 0;
+        }
         while ((*nl) != NULL) {
                 if ((*nl) == n)
                         return 0;
@@ -82,6 +90,14 @@ static int __kprobes notifier_call_chain(struct notifier_block **nl,
 
         while (nb && nr_to_call) {
                 next_nb = rcu_dereference(nb->next);
+
+#ifdef CONFIG_DEBUG_NOTIFIERS
+                if (!kernel_text_address((unsigned long)nb->notifier_call)) {
+                        WARN(1, "Invalid notifier called!");
+                        nb = next_nb;
+                        continue;
+                }
+#endif
                 ret = nb->notifier_call(nb, val, v);
 
                 if (nr_calls)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8b5a7d304a5f..342858fbabbc 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -536,6 +536,16 @@ config DEBUG_SG
 
           If unsure, say N.
 
+config DEBUG_NOTIFIERS
+        bool "Debug notifier call chains"
+        depends on DEBUG_KERNEL
+        help
+          Enable this to turn on sanity checking for notifier call chains.
+          This is most useful for kernel developers to make sure that
+          modules properly unregister themselves from notifier chains.
+          This is a relatively cheap check but if you care about maximum
+          performance, say N.
+
 config FRAME_POINTER
         bool "Compile the kernel with frame pointers"
         depends on DEBUG_KERNEL && \