[PATCH bpf] bpf: Reject negative optlen in cgroup getsockopt hook

[Junseo Lim <zirajs7@gmail.com>]

A cgroup getsockopt BPF program can shrink ctx->optlen after the
kernel getsockopt handler has run. The kernel-buffer variant, used by
TCP_ZEROCOPY_RECEIVE, only rejects values larger than the original
length.

If BPF writes a negative optlen, that value is accepted and propagated
back to the TCP getsockopt code. It can then be passed to
copy_to_sockptr() as a size_t and trigger the hardened usercopy
bytes > INT_MAX warning.

Reject negative ctx.optlen in __cgroup_bpf_run_filter_getsockopt_kern(),
matching the lower-bound validation already present in the sockptr-based
getsockopt hook.

Fixes: 9cacf81f8161 ("bpf: Remove extra lock_sock for TCP_ZEROCOPY_RECEIVE")
Signed-off-by: Junseo Lim <zirajs7@gmail.com>
---
Reproducer and warning: 
https://gist.github.com/ZirAjs/a177ec6d8f2c8ed7d93edca6313a9255

Tested by building and booting the patched kernel. The reproducer 
returns -EFAULT and no longer triggers the hardened usercopy warning.

 kernel/bpf/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 4355ccb78a9c..c04a244fe2e6 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -2235,7 +2235,7 @@ int __cgroup_bpf_run_filter_getsockopt_kern(struct sock *sk, int level,
 	if (ret < 0)
 		return ret;
 
-	if (ctx.optlen > *optlen)
+	if (ctx.optlen > *optlen || ctx.optlen < 0)
 		return -EFAULT;
 
 	/* BPF programs can shrink the buffer, export the modifications.
-- 
2.55.0