A toctou issue in `__cgroup_bpf_run_filter_getsockopt` can trigger a
WARN_ON_ONCE in a check of `copy_from_user`.
`*optlen` is checked to be non-negative in the individual getsockopt
functions beforehand. Changing `*optlen` in a race to a negative value
will result in a `copy_from_user(ctx.optval, optval, ctx.optlen)` with
`ctx.optlen` being a negative integer.

Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Signed-off-by: Loris Reiff <redacted>
---
 kernel/bpf/cgroup.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 96555a8a2..6ec8f02f4 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c

@@ -1442,6 +1442,11 @@ int __cgroup_bpf_run_filter_getsockopt(struct sock *sk, int level,
                        goto out;
                }

+               if (ctx.optlen < 0) {
+                       ret = -EFAULT;
+                       goto out;
+               }
+
                if (copy_from_user(ctx.optval, optval,
                                   min(ctx.optlen, max_optlen)) != 0) {
                        ret = -EFAULT;
--

2.29.2