Re: [PATCH net v4] bpf, net: Check cgroup_bpf_enabled() only once in do_sock_getsockopt()

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()` was 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)

To prevent this, invoke `cgroup_bpf_enabled()` only once and cache the
result in a newly added local variable `enabled`.
Both `BPF_CGROUP_*` macros in `do_sock_getsockopt` will then check their
condition using the same `enabled` variable as the condition variable,
instead of using the return values from `cgroup_bpf_enabled` called by
themselves as the condition variable(which could yield different results).
This ensures that either both `BPF_CGROUP_*` macros pass the condition
or neither does.

Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Co-developed-by: Yanghui Li <redacted>
Signed-off-by: Yanghui Li <redacted>
Co-developed-by: Cheng-Jui Wang <redacted>
Signed-off-by: Cheng-Jui Wang <redacted>
Signed-off-by: Tze-nan Wu <redacted>
---

Chagnes from v1 to v2: https://lore.kernel.org/all/20240819082513.27176-1-Tze-nan.Wu@mediatek.com/ (local)
  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.

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

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

---
 include/linux/bpf-cgroup.h | 15 ++++++++-------
 net/socket.c               |  5 +++--
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index fb3c3e7181e6..5afa2ac76aae 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h

@@ -390,20 +390,20 @@ static inline bool cgroup_bpf_sock_enabled(struct sock *sk,
        __ret;                                                                 \
 })

-#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen)                              \
+#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, enabled)                     \
 ({                                                                            \
        int __ret = 0;                                                         \
-       if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT))                             \
+       enabled = cgroup_bpf_enabled(CGROUP_GETSOCKOPT);                       \
+       if (enabled)