[PATCH] net/socket: Acquire cgroup_lock in do_sock_getsockopt

[Tze-nan Wu]

The return value from `cgroup_bpf_enabled(CGROUP_GETSOCKOPT)` can change
between the invocations of `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN` and
`BPF_CGROUP_RUN_PROG_GETSOCKOPT`.

If `cgroup_bpf_enabled(CGROUP_GETSOCKOPT)` changes from "false" to
"true"
between the invocations of `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN` and
`BPF_CGROUP_RUN_PROG_GETSOCKOPT`,
`BPF_CGROUP_RUN_PROG_GETSOCKOPT` will receive an -EFAULT from
`__cgroup_bpf_run_filter_getsockopt(max_optlen=0)` due to `get_user()`
had not reached in `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN`.

Scenario shown as below:

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

Prevent `cgroup_bpf_enabled(CGROUP_GETSOCKOPT)` change between
`BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN` and `BPF_CGROUP_RUN_PROG_GETSOCKOPT`
by acquiring cgroup_lock.

Co-developed-by: Yanghui Li <yanghui.li@xxxxxxxxxxxx>
Signed-off-by: Yanghui Li <yanghui.li@xxxxxxxxxxxx>
Co-developed-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx>
Signed-off-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>

---

We have encountered this issue by observing that process A could sometimes
get an -EFAULT from getsockopt() during our device boot-up, while another
process B triggers the race condition by enabling CGROUP_GETSOCKOPT
through bpf syscall at the same time.

The race condition is shown below:

           `process A`                        `process B`
           -----------                        ------------
  BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN
         
                                              bpf syscall 
                                        (CGROUP_GETSOCKOPT enabled)

  BPF_CGROUP_RUN_PROG_GETSOCKOPT
  -> __cgroup_bpf_run_filter_getsockopt
    (-EFAULT)

__cgroup_bpf_run_filter_getsockopt return -EFAULT at the line shown below:
	if (optval && (ctx.optlen > max_optlen || ctx.optlen < 0)) {
		if (orig_optlen > PAGE_SIZE && ctx.optlen >= 0) {
			pr_info_once("bpf getsockopt: ignoring program buffer with optlen=%d (max_optlen=%d)\n",
				     ctx.optlen, max_optlen);
			ret = retval;
			goto out;
		}
		ret = -EFAULT; <== return EFAULT here
		goto out;
	}

This patch should fix the race but not sure if it introduces any potential
side effects or regression.

And we wondering if this is a real issue in do_sock_getsockopt or if
getsockopt() is designed to expect such race conditions.
Should the userspace caller always anticipate an -EFAULT from getsockopt()
if another process enables CGROUP_GETSOCKOPT at the same time?

Any comment will be appreciated!

BTW, I added Chengjui and Yanghui to Co-developed due to we have several
discussions on this issue. And we both spend some time on this issue.

---
 net/socket.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index fcbdd5bc47ac..e0b2b16fd238 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2370,8 +2370,10 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
 	if (err)
 		return err;
 
-	if (!compat)
+	if (!compat) {
+		cgroup_lock();
 		max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);
+	}
 
 	ops = READ_ONCE(sock->ops);
 	if (level == SOL_SOCKET) {
@@ -2387,10 +2389,12 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
 				      optlen.user);
 	}
 
-	if (!compat)
+	if (!compat) {
 		err = BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock->sk, level, optname,
 						     optval, optlen, max_optlen,
 						     err);
+		cgroup_unlock();
+	}
 
 	return err;
 }
-- 
2.45.2