[PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

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

In fprobe_return(), the shadow-stack iteration and exit_handler
invocations were protected by preempt_disable_notrace().
However, 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 the fprobe is no longer in use before
freeing it.

In preemptible kernels (CONFIG_PREEMPT_RCU=y), standard RCU grace
periods do not wait for pure preempt_disable_notrace() critical
sections. Consequently, an unregistered fprobe may be freed while
a concurrent CPU executing fprobe_return() is still running
fp->exit_handler(), causing a use-after-free.

To resolve this, protect fprobe_return() with guard(rcu)() matching
fprobe_fgraph_entry(). This ensures both synchronous unregister_fprobe()
and asynchronous unregister_fprobe_async() safely wait for in-flight
exit_handlers to complete via standard RCU grace periods.

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 |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 9f2d98181779..c3e1580bc648 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c

@@ -671,7 +671,7 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 	size_words = SIZE_IN_LONG(size);
 	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
 
-	preempt_disable_notrace();
+	guard(rcu)();
 
 	curr = 0;
 	while (size_words > curr) {

@@ -687,7 +687,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 		}
 		curr += size;
 	}
-	preempt_enable_notrace();
 }
 NOKPROBE_SYMBOL(fprobe_return);