fgraph: Add READ_ONCE() when accessing fgraph_array[]

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: fgraph: Add READ_ONCE() when accessing fgraph_array[] In __ftrace_return_to_handler(), a loop iterates over the fgraph_array[] elements, which are fgraph_ops. The loop checks if an element is a fgraph_stub to prevent using a fgraph_stub afterward. However, if the compiler reloads fgraph_array[] after this check, it might race with an update to fgraph_array[] that introduces a fgraph_stub. This could result in the stub being processed, but the stub contains a null "func_hash" field, leading to a NULL pointer dereference. To ensure that the gops compared against the fgraph_stub matches the gops processed later, add a READ_ONCE(). A similar patch appears in commit 63a8dfb ("function_graph: Add READ_ONCE() when accessing fgraph_array[]").

References

• https://git.kernel.org/stable/c/b68b2a3fbacc7be720ef589d489bcacdd05c6d38
• https://git.kernel.org/stable/c/d65474033740ded0a4fe9a097fce72328655b41d