[PATCH] tracing: fgraph: Protect return handler from recursion loop

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

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

function_graph_enter_regs() prevents itself from recursion by
ftrace_test_recursion_trylock(), but __ftrace_return_to_handler(),
which is called at the exit, does not prevent such recursion.
Therefore, while it can prevent recursive calls from
fgraph_ops::entryfunc(), it is not able to prevent recursive calls
to fgraph from fgraph_ops::retfunc(), resulting in a recursive loop.
This can lead an unexpected recursion bug reported by Menglong.

 is_endbr() is called in __ftrace_return_to_handler -> fprobe_return
  -> kprobe_multi_link_exit_handler -> is_endbr.

To fix this issue, acquire ftrace_test_recursion_trylock() in the
__ftrace_return_to_handler() after unwind the shadow stack to mark
this section must prevent recursive call of fgraph inside user-defined
fgraph_ops::retfunc().

This is essentially a fix to commit 4346ba160409 ("fprobe: Rewrite
fprobe on function-graph tracer"), because before that fgraph was
only used from the function graph tracer. Fprobe allowed user to run
any callbacks from fgraph after that commit.

Reported-by: Menglong Dong <menglong8.dong@gmail.com>
Closes: https://lore.kernel.org/all/20250918120939.1706585-1-dongml2@chinatelecom.cn/
Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/fgraph.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 1e3b32b1e82c..08dde420635b 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -815,6 +815,7 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
 	unsigned long bitmap;
 	unsigned long ret;
 	int offset;
+	int bit;
 	int i;
 
 	ret_stack = ftrace_pop_return_trace(&trace, &ret, frame_pointer, &offset);
@@ -829,6 +830,15 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
 	if (fregs)
 		ftrace_regs_set_instruction_pointer(fregs, ret);
 
+	bit = ftrace_test_recursion_trylock(trace.func, ret);
+	/*
+	 * This must be succeeded because the entry handler returns before
+	 * modifying the return address if it is nested. Anyway, we need to
+	 * avoid calling user callbacks if it is nested.
+	 */
+	if (WARN_ON_ONCE(bit < 0))
+		goto out;
+
 #ifdef CONFIG_FUNCTION_GRAPH_RETVAL
 	trace.retval = ftrace_regs_get_return_value(fregs);
 #endif
@@ -852,6 +862,8 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
 		}
 	}
 
+	ftrace_test_recursion_unlock(bit);
+out:
 	/*
 	 * The ftrace_graph_return() may still access the current
 	 * ret_stack structure, we need to make sure the update of