bpf, net: Fix a potential race in do_sock_getsockopt()

There's a potential race when `cgroup_bpf_enabled(CGROUP_GETSOCKOPT)` is
false during the execution of `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN`, but
becomes true when `BPF_CGROUP_RUN_PROG_GETSOCKOPT` is called.
This inconsistency can lead to `BPF_CGROUP_RUN_PROG_GETSOCKOPT` receiving
an "-EFAULT" from `__cgroup_bpf_run_filter_getsockopt(max_optlen=0)`.
Scenario shown as below:

           `process A`                      `process B`
           -----------                      ------------
  BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN
                                            enable CGROUP_GETSOCKOPT
  BPF_CGROUP_RUN_PROG_GETSOCKOPT (-EFAULT)

To resolve this, remove the `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN` macro and
directly uses `copy_from_sockptr` to ensure that `max_optlen` is always
set before `BPF_CGROUP_RUN_PROG_GETSOCKOPT` is invoked.

Fixes: 0d01da6 ("bpf: implement getsockopt and setsockopt hooks")
Co-developed-by: Yanghui Li <yanghui.li@mediatek.com>
Signed-off-by: Yanghui Li <yanghui.li@mediatek.com>
Co-developed-by: Cheng-Jui Wang <cheng-jui.wang@mediatek.com>
Signed-off-by: Cheng-Jui Wang <cheng-jui.wang@mediatek.com>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@mediatek.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20240830082518.23243-1-Tze-nan.Wu@mediatek.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Tze-nan Wu

include/linux/bpf-cgroup.h

@@ -390,14 +390,6 @@ static inline bool cgroup_bpf_sock_enabled(struct sock *sk,
 	__ret;								       \
 })
 
-#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen)			       \
-({									       \
-	int __ret = 0;							       \
-	if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT))			       \
-		copy_from_sockptr(&__ret, optlen, sizeof(int));		       \
-	__ret;								       \
-})
-
 #define BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock, level, optname, optval, optlen,   \
 				       max_optlen, retval)		       \
 ({									       \
@@ -518,7 +510,6 @@ static inline int bpf_percpu_cgroup_storage_update(struct bpf_map *map,
 #define BPF_CGROUP_RUN_PROG_SOCK_OPS(sock_ops) ({ 0; })
 #define BPF_CGROUP_RUN_PROG_DEVICE_CGROUP(atype, major, minor, access) ({ 0; })
 #define BPF_CGROUP_RUN_PROG_SYSCTL(head,table,write,buf,count,pos) ({ 0; })
-#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen) ({ 0; })
 #define BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock, level, optname, optval, \
 				       optlen, max_optlen, retval) ({ retval; })
 #define BPF_CGROUP_RUN_PROG_GETSOCKOPT_KERN(sock, level, optname, optval, \

net/socket.c

@@ -2362,7 +2362,7 @@ INDIRECT_CALLABLE_DECLARE(bool tcp_bpf_bypass_getsockopt(int level,
 int do_sock_getsockopt(struct socket *sock, bool compat, int level,
 		       int optname, sockptr_t optval, sockptr_t optlen)
 {
-	int max_optlen __maybe_unused;
+	int max_optlen __maybe_unused = 0;
 	const struct proto_ops *ops;
 	int err;
 
@@ -2371,7 +2371,7 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
 		return err;
 
 	if (!compat)
-		max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);
+		copy_from_sockptr(&max_optlen, optlen, sizeof(int));
 
 	ops = READ_ONCE(sock->ops);
 	if (level == SOL_SOCKET) {