[PATCH 2/8] kprobes: Fix double free and NULL deref in unregister_kretprobes()

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

In unregister_kretprobes(), if __unregister_kprobe_top() returns an error
(e.g., if the kretprobe was not registered or was already unregistered),
rethook_free(rps[i]->rh) or rcu_assign_pointer(rps[i]->rph->rp, NULL) is
still executed. For rethook-based kretprobes, calling rethook_free() on an
already-freed rethook causes a double-free / list corruption in RCU. For
legacy kretprobes, rps[i]->rph is NULL after previous cleanup, resulting
in a NULL pointer dereference.

Skip rethook_free() and rph cleanup if __unregister_kprobe_top() fails,
and clear rps[i]->rh after synchronize_rcu() has waited for in-flight
pre_handler_kretprobe() invocations to complete.

Fixes: 73f9b911faa7 ("kprobes: Use rethook for kretprobe if possible")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/kprobes.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 10946658bb35..7a3da809bac7 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -2605,8 +2605,10 @@ void unregister_kretprobes(struct kretprobe **rps, int num)
 	for (i = 0; i < num; i++) {
 		guard(mutex)(&kprobe_mutex);
 
-		if (__unregister_kprobe_top(&rps[i]->kp) < 0)
+		if (__unregister_kprobe_top(&rps[i]->kp) < 0) {
 			rps[i]->kp.addr = NULL;
+			continue;
+		}
 #ifdef CONFIG_KRETPROBE_ON_RETHOOK
 		rethook_free(rps[i]->rh);
 #else
@@ -2618,7 +2620,9 @@ void unregister_kretprobes(struct kretprobe **rps, int num)
 	for (i = 0; i < num; i++) {
 		if (rps[i]->kp.addr) {
 			__unregister_kprobe_bottom(&rps[i]->kp);
-#ifndef CONFIG_KRETPROBE_ON_RETHOOK
+#ifdef CONFIG_KRETPROBE_ON_RETHOOK
+			rps[i]->rh = NULL;
+#else
 			free_rp_inst(rps[i]);
 #endif
 		}