[PATCH] function_graph: Add READ_ONCE() when accessing fgraph_array[]

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

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@xxxxxxxxxxxxxx/

Fixes: cc60ee813b503 ("function_graph: Use static_call and branch to optimize entry function")
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
 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