[PATCH net-next v5 01/14] net: Block MSG_SENDPAGE_* from being passed to sendmsg() by userspace

From: David Howells
Date: Wed Jun 07 2023 - 10:32:02 EST


It is necessary to allow MSG_SENDPAGE_* to be passed into ->sendmsg() to
allow sendmsg(MSG_SPLICE_PAGES) to replace ->sendpage(). Unblocking them
in the network protocol, however, allows these flags to be passed in by
userspace too[1].

Fix this by marking MSG_SENDPAGE_NOPOLICY, MSG_SENDPAGE_NOTLAST and
MSG_SENDPAGE_DECRYPTED as internal flags, which causes sendmsg() to object
if they are passed to sendmsg() by userspace. Network protocol ->sendmsg()
implementations can then allow them through.

Note that it should be possible to remove MSG_SENDPAGE_NOTLAST once
sendpage is removed as a whole slew of pages will be passed in in one go by
splice through sendmsg, with MSG_MORE being set if it has more data waiting
in the pipe.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Jakub Kicinski <kuba@xxxxxxxxxx>
cc: Chuck Lever <chuck.lever@xxxxxxxxxx>
cc: Boris Pismenny <borisp@xxxxxxxxxx>
cc: John Fastabend <john.fastabend@xxxxxxxxx>
cc: Eric Dumazet <edumazet@xxxxxxxxxx>
cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
cc: Paolo Abeni <pabeni@xxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: netdev@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20230526181338.03a99016@xxxxxxxxxx/ [1]
---
include/linux/socket.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index bd1cc3238851..3fd3436bc09f 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -339,7 +339,9 @@ struct ucred {
#endif

/* Flags to be cleared on entry by sendmsg and sendmmsg syscalls */
-#define MSG_INTERNAL_SENDMSG_FLAGS (MSG_SPLICE_PAGES)
+#define MSG_INTERNAL_SENDMSG_FLAGS \
+ (MSG_SPLICE_PAGES | MSG_SENDPAGE_NOPOLICY | MSG_SENDPAGE_NOTLAST | \
+ MSG_SENDPAGE_DECRYPTED)

/* Setsockoptions(2) level. Thanks to BSD these must match IPPROTO_xxx */
#define SOL_IP 0