[PATCH 5.15.y] kprobes: Protect kprobe_blacklist with RCU

[Sasha Levin <sashal@kernel.org>]

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

[ Upstream commit 0c4256196b3a105307e2235fbfd85e768bbcdd0f ]

__within_kprobe_blacklist() traverses kprobe_blacklist without holding
kprobe_mutex. When a module is unloaded, kprobe_remove_area_blacklist()
removes blacklist entries and immediately frees them with kfree().
A concurrent call to within_kprobe_blacklist() can therefore dereference
freed memory.

Furthermore, within_kprobe_blacklist() can be called in atomic or
non-preemptible contexts where the sleeping kprobe_mutex cannot be taken.

Protect kprobe_blacklist with RCU. Use guard(rcu)() and
list_for_each_entry_rcu() for traversal, list_add_tail_rcu() for
insertions, list_del_rcu() for deletions, and kfree_rcu() to reclaim
entries safely after a grace period.

Link: https://lore.kernel.org/all/178810004323.64882.16493230858653316962.stgit@devnote2/

Fixes: 376e242429bf ("kprobes: Introduce NOKPROBE_SYMBOL() macro to maintain kprobes blacklist")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260807155802.F06041F000E9@smtp.kernel.org/
Assisted-by: Antigravity:gemini-3.7-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
[ Replaced guard(rcu)() with explicit rcu_read_lock()/rcu_read_unlock() to avoid declaration-after-statement warnings. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/kprobes.h |  1 +
 kernel/kprobes.c        | 19 ++++++++++++++-----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 24b0eaa5de307..e5262feff8267 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -177,6 +177,7 @@ struct kprobe_blacklist_entry {
 	struct list_head list;
 	unsigned long start_addr;
 	unsigned long end_addr;
+	struct rcu_head rcu;
 };
 
 #ifdef CONFIG_KPROBES
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 0d463859ad329..69d166f42d04a 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1427,11 +1427,20 @@ static bool __within_kprobe_blacklist(unsigned long addr)
 	/*
 	 * If there exists a kprobe_blacklist, verify and
 	 * fail any probe registration in the prohibited area
+	 * Note: this can return true during transition period where
+	 * (start_addr, end_addr) in the black list is shrinking
+	 * but old entry has not been removed yet. This is acceptable
+	 * because the worst case is that we reject more probes than
+	 * we should.
 	 */
-	list_for_each_entry(ent, &kprobe_blacklist, list) {
-		if (addr >= ent->start_addr && addr < ent->end_addr)
+	rcu_read_lock();
+	list_for_each_entry_rcu(ent, &kprobe_blacklist, list) {
+		if (addr >= ent->start_addr && addr < ent->end_addr) {
+			rcu_read_unlock();
 			return true;
+		}
 	}
+	rcu_read_unlock();
 	return false;
 }
 
@@ -2258,7 +2267,7 @@ int kprobe_add_ksym_blacklist(unsigned long entry)
 	ent->start_addr = entry;
 	ent->end_addr = entry + size;
 	INIT_LIST_HEAD(&ent->list);
-	list_add_tail(&ent->list, &kprobe_blacklist);
+	list_add_tail_rcu(&ent->list, &kprobe_blacklist);
 
 	return (int)size;
 }
@@ -2287,8 +2296,8 @@ static void kprobe_remove_area_blacklist(unsigned long start, unsigned long end)
 	list_for_each_entry_safe(ent, n, &kprobe_blacklist, list) {
 		if (ent->start_addr < start || ent->start_addr >= end)
 			continue;
-		list_del(&ent->list);
-		kfree(ent);
+		list_del_rcu(&ent->list);
+		kfree_rcu(ent, rcu);
 	}
 }
 
-- 
2.53.0