sk-filter: Add ability to lock a socket filter program

While a privileged program can open a raw socket, attach some
restrictive filter and drop its privileges (or send the socket to an
unprivileged program through some Unix socket), the filter can still
be removed or modified by the unprivileged program. This commit adds a
socket option to lock the filter (SO_LOCK_FILTER) preventing any
modification of a socket filter program.

This is similar to OpenBSD BIOCLOCK ioctl on bpf sockets, except even
root is not allowed change/drop the filter.

The state of the lock can be read with getsockopt(). No error is
triggered if the state is not changed. -EPERM is returned when a user
tries to remove the lock or to change/remove the filter while the lock
is active. The check is done directly in sk_attach_filter() and
sk_detach_filter() and does not affect only setsockopt() syscall.

Signed-off-by: Vincent Bernat <bernat@luffy.cx>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 3 insertions, 0 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index 182ca99405ad..5a34e2f03657 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -664,6 +664,7 @@ enum sock_flags {
                      * Will use last 4 bytes of packet sent from
                      * user-space instead.
                      */
+        SOCK_FILTER_LOCKED, /* Filter cannot be changed anymore */
 };
 
 static inline void sock_copy_flags(struct sock *nsk, struct sock *osk)
diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index 2d32d073a6f9..3f6a99201410 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h
@@ -73,4 +73,6 @@
 /* Instruct lower device to use last 4-bytes of skb data as FCS */
 #define SO_NOFCS                43
 
+#define SO_LOCK_FILTER          44
+
 #endif /* __ASM_GENERIC_SOCKET_H */