[PATCH] fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()

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

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

unregister_fprobe() and unregister_fprobe_async() (used by BPF
kprobe-multi) rely on standard RCU grace periods (synchronize_rcu()
and call_rcu()) to wait until in-flight fprobe handlers complete before
freeing the fprobe.

However, if an fprobe handler executes while RCU is not watching (such
as in the idle loop or nohz_full extended quiescent states), standard
RCU does not track preemption-disabled sections. Consequently,
synchronize_rcu() does not wait for those executions, which can lead
to a use-after-free if the fprobe is freed immediately after
unregistration. Ensure handlers exit early when !rcu_is_watching().

Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously
used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every
hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep
recursion if probed functions interact with lockdep.

Since rhltable_lookup() and rhl_for_each_entry_rcu() use
rcu_dereference_all_check() (which checks rcu_read_lock_any_held()),
holding preemption disabled via rcu_read_lock_sched_notrace() is fully
valid and sufficient so long as rcu_is_watching() is true.

Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(),
fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path
rcu_read_lock() and lockdep overhead while guaranteeing safe grace
period synchronization.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3
Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace period")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/fprobe.c |   26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 9f2d98181779..da286619c5d8 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -47,6 +47,10 @@ static struct rhltable fprobe_ip_table;
 static DEFINE_MUTEX(fprobe_mutex);
 static struct fgraph_ops fprobe_graph_ops;
 
+DEFINE_LOCK_GUARD_0(rcu_sched_notrace,
+		    rcu_read_lock_sched_notrace(),
+		    rcu_read_unlock_sched_notrace())
+
 static u32 fprobe_node_hashfn(const void *data, u32 len, u32 seed)
 {
 	return hash_ptr(*(unsigned long **)data, 32);
@@ -329,16 +333,14 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
 	struct fprobe *fp;
 	int bit;
 
+	if (!rcu_is_watching())
+		return;
+
 	bit = ftrace_test_recursion_trylock(ip, parent_ip);
 	if (bit < 0)
 		return;
 
-	/*
-	 * ftrace_test_recursion_trylock() disables preemption, but
-	 * rhltable_lookup() checks whether rcu_read_lcok is held.
-	 * So we take rcu_read_lock() here.
-	 */
-	rcu_read_lock();
+	guard(rcu_sched_notrace)();
 	head = rhltable_lookup(&fprobe_ip_table, &ip, fprobe_rht_params);
 
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {
@@ -353,7 +355,6 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
 		else
 			__fprobe_handler(ip, parent_ip, fp, fregs, NULL);
 	}
-	rcu_read_unlock();
 	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(fprobe_ftrace_entry);
@@ -567,10 +568,13 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 	struct fprobe *fp;
 	int used, ret;
 
+	if (!rcu_is_watching())
+		return 0;
+
 	if (WARN_ON_ONCE(!fregs))
 		return 0;
 
-	guard(rcu)();
+	guard(rcu_sched_notrace)();
 	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
 	reserved_words = 0;
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {
@@ -665,13 +669,16 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 	int size, curr;
 	int size_words;
 
+	if (!rcu_is_watching())
+		return;
+
 	fgraph_data = (unsigned long *)fgraph_retrieve_data(gops->idx, &size);
 	if (WARN_ON_ONCE(!fgraph_data))
 		return;
 	size_words = SIZE_IN_LONG(size);
 	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
 
-	preempt_disable_notrace();
+	guard(rcu_sched_notrace)();
 
 	curr = 0;
 	while (size_words > curr) {
@@ -687,7 +694,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 		}
 		curr += size;
 	}
-	preempt_enable_notrace();
 }
 NOKPROBE_SYMBOL(fprobe_return);