linux-kernel - [PATCH] function_graph: Add READ_ONCE() when accessing fgraph

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Message-ID: <20240613095223.1f07e3a4@rorschach.local.home>
Date: Thu, 13 Jun 2024 09:52:23 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, Linux trace kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland
 <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: [PATCH] function_graph: Add READ_ONCE() when accessing
 fgraph_array[]

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

In function_graph_enter() there's a loop that looks at fgraph_array[]
elements which are fgraph_ops. It first tests if it is a fgraph_stub op,
and if so skips it, as that's just there as a place holder. Then it checks
the fgraph_ops filters to see if the ops wants to trace the current
function.

But if the compiler reloads the fgraph_array[] after the check against
fgraph_stub, it could race with the fgraph_array[] being updated with the
fgraph_stub. That would cause the stub to be processed. But the stub has a
null "func_hash" field which will cause a NULL pointer dereference.

Add a READ_ONCE() so that the gops that is compared against the
fgraph_stub is also the gops that is processed later.

Link: https://lore.kernel.org/all/CA+G9fYsSVJQZH=nM=1cjTc94PgSnMF9y65BnOv6XSoCG_b6wmw@mail.gmail.com/

Fixes: cc60ee813b503 ("function_graph: Use static_call and branch to optimize entry function")
Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/fgraph.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 8317d1a7f43a..fc205ad167a9 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -641,7 +641,7 @@ int function_graph_enter(unsigned long ret, unsigned long func,
 	{
 		for_each_set_bit(i, &fgraph_array_bitmask,
 					 sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
-			struct fgraph_ops *gops = fgraph_array[i];
+			struct fgraph_ops *gops = READ_ONCE(fgraph_array[i]);
 			int save_curr_ret_stack;

 			if (gops == &fgraph_stub)
-- 
2.43.0