[PATCH 4/5] bpf: sockmap, sockhash: return file descriptors from privileged lookup

[Lorenz Bauer <lmb@cloudflare.com>]

Allow callers with CAP_NET_ADMIN to retrieve file descriptors from a
sockmap and sockhash. O_CLOEXEC is enforced on all fds.

Without this, it's difficult to resize or otherwise rebuild existing
sockmap or sockhashes.

Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
---
 net/core/sock_map.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index 03e04426cd21..3228936aa31e 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -347,12 +347,31 @@ static void *sock_map_lookup(struct bpf_map *map, void *key)
 static int __sock_map_copy_value(struct bpf_map *map, struct sock *sk,
 				 void *value)
 {
+	struct file *file;
+	int fd;
+
 	switch (map->value_size) {
 	case sizeof(u64):
 		sock_gen_cookie(sk);
 		*(u64 *)value = atomic64_read(&sk->sk_cookie);
 		return 0;
 
+	case sizeof(u32):
+		if (!capable(CAP_NET_ADMIN))
+			return -EPERM;
+
+		fd = get_unused_fd_flags(O_CLOEXEC);
+		if (unlikely(fd < 0))
+			return fd;
+
+		read_lock_bh(&sk->sk_callback_lock);
+		file = get_file(sk->sk_socket->file);
+		read_unlock_bh(&sk->sk_callback_lock);
+
+		fd_install(fd, file);
+		*(u32 *)value = fd;
+		return 0;
+
 	default:
 		return -ENOSPC;
 	}
-- 
2.20.1