[PATCH net v5] bpf, net: Fix a potential race in do_sock_getsockopt()

[Tze-nan Wu]

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: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Co-developed-by: Yanghui Li <yanghui.li at mediatek.com>
Signed-off-by: Yanghui Li <yanghui.li at mediatek.com>
Co-developed-by: Cheng-Jui Wang <cheng-jui.wang at mediatek.com>
Signed-off-by: Cheng-Jui Wang <cheng-jui.wang at mediatek.com>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu at mediatek.com>
---

Changes from v1 to v2: https://lore.kernel.org/all/20240819082513.27176-1-Tze-nan.Wu@mediatek.com/
  Instead of using cgroup_lock in the fastpath, invoke cgroup_bpf_enabled
  only once and cache the value in the newly added variable `enabled`.
  `BPF_CGROUP_*` macros in do_sock_getsockopt can then both check their
  condition with the new variable `enable`, ensuring that either they both
  passing the condition or both do not.

Changes from v2 to v3: https://lore.kernel.org/all/20240819155627.1367-1-Tze-nan.Wu@mediatek.com/
  Hide cgroup_bpf_enabled in the macro, and some modifications to adapt
  the coding style.

Changes from v3 to v4: https://lore.kernel.org/all/20240820092942.16654-1-Tze-nan.Wu@mediatek.com/
  Add bpf tag to subject, and add the Fixes tag in body.

Changes from v4 to v5: https://lore.kernel.org/all/20240821093016.2533-1-Tze-nan.Wu@mediatek.com/
  Change subject, and fix the race by unconditional `copy_from_sockptr`,
  instead of adding a new variable.

---
 include/linux/bpf-cgroup.h | 9 ---------
 net/socket.c               | 4 ++--
 2 files changed, 2 insertions(+), 11 deletions(-)

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index fb3c3e7181e6..ce91d9b2acb9 100644
--- a/include/linux/bpf-cgroup.h
+++ b/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, \
diff --git a/net/socket.c b/net/socket.c
index fcbdd5bc47ac..0a2bd22ec105 100644
--- a/net/socket.c
+++ b/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) {
-- 
2.45.2