net: block MSG_NO_SHARED_FRAGS in sendmsg()

This change should cause no difference in behavior; it just cleans up some
hazardous code that could have become a problem in the future.

MSG_NO_SHARED_FRAGS is a kernel-internal flag that cancels the effect of
MSG_SPLICE_PAGES, another kernel-internal flag that influences the
data-sharing semantics of SKBs.

Prevent passing this flag in from userspace via sendmsg() by adding it to
MSG_INTERNAL_SENDMSG_FLAGS.

This is not currently an observable problem because MSG_NO_SHARED_FRAGS
only has an effect if kernel code adds MSG_SPLICE_PAGES to it.
The only codepath that adds MSG_SPLICE_PAGES to user-supplied flags from
which MSG_NO_SHARED_FRAGS hasn't been cleared is the path
tcp_bpf_sendmsg -> tcp_bpf_send_verdict -> tcp_bpf_push, and that is not a
problem because tcp_bpf_sendmsg always intentionally sets
MSG_NO_SHARED_FRAGS anyway.

Signed-off-by: Jann Horn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

include/linux/socket.h

@@ -357,7 +357,7 @@ struct ucred {
 
 /* Flags to be cleared on entry by sendmsg and sendmmsg syscalls */
 #define MSG_INTERNAL_SENDMSG_FLAGS \
-	(MSG_SPLICE_PAGES | MSG_SENDPAGE_NOPOLICY | MSG_SENDPAGE_DECRYPTED)
+	(MSG_SPLICE_PAGES | MSG_SENDPAGE_NOPOLICY | MSG_SENDPAGE_DECRYPTED | MSG_NO_SHARED_FRAGS)
 
 /* Setsockoptions(2) level. Thanks to BSD these must match IPPROTO_xxx */
 #define SOL_IP		0