[PATCH v2] kprobes: avoid the kprobe being re-registered

[Zhou Chengming]

Changes from v1:
- We should put the modifies of the kprobe after the re-reg check.
- And then the address_safe check.
- When check_kprobe_address_safe() return fail, the *probed_mod
  should be set to NULL, and no module refcount held.

Old code use check_kprobe_rereg() to check if the kprobe has been
registered already, but check_kprobe_rereg() will release the
kprobe_mutex then, so maybe two paths will pass the check and
register the same kprobe. This patch put the check inside the mutex.

Signed-off-by: Zhou Chengming <zhouchengming1@xxxxxxxxxx>
---
 kernel/kprobes.c | 28 +++++++++-------------------
 1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index a1606a4..f622639 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1443,19 +1443,6 @@ static struct kprobe *__get_valid_kprobe(struct kprobe *p)
 	return ap;
 }
 
-/* Return error if the kprobe is being re-registered */
-static inline int check_kprobe_rereg(struct kprobe *p)
-{
-	int ret = 0;
-
-	mutex_lock(&kprobe_mutex);
-	if (__get_valid_kprobe(p))
-		ret = -EINVAL;
-	mutex_unlock(&kprobe_mutex);
-
-	return ret;
-}
-
 int __weak arch_check_ftrace_location(struct kprobe *p)
 {
 	unsigned long ftrace_addr;
@@ -1501,6 +1488,7 @@ static int check_kprobe_address_safe(struct kprobe *p,
 		 * its code to prohibit unexpected unloading.
 		 */
 		if (unlikely(!try_module_get(*probed_mod))) {
+			*probed_mod = NULL;
 			ret = -ENOENT;
 			goto out;
 		}
@@ -1536,9 +1524,13 @@ int register_kprobe(struct kprobe *p)
 		return PTR_ERR(addr);
 	p->addr = addr;
 
-	ret = check_kprobe_rereg(p);
-	if (ret)
-		return ret;
+	mutex_lock(&kprobe_mutex);
+
+	/* Return error if the kprobe is being re-registered */
+	if (__get_valid_kprobe(p)) {
+		ret = -EINVAL;
+		goto out;
+	}
 
 	/* User can pass only KPROBE_FLAG_DISABLED to register_kprobe */
 	p->flags &= KPROBE_FLAG_DISABLED;
@@ -1547,9 +1539,7 @@ int register_kprobe(struct kprobe *p)
 
 	ret = check_kprobe_address_safe(p, &probed_mod);
 	if (ret)
-		return ret;
-
-	mutex_lock(&kprobe_mutex);
+		goto out;
 
 	old_p = get_kprobe(p->addr);
 	if (old_p) {
-- 
1.8.3.1