sctp: fix -ENOMEM result with invalid user space pointer in sendto() syscall

Consider the following program, that sets the second argument to the
sendto() syscall incorrectly:

 #include <string.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>

 int main(void)
 {
         int fd;
         struct sockaddr_in sa;

         fd = socket(AF_INET, SOCK_STREAM, 132 /*IPPROTO_SCTP*/);
         if (fd < 0)
                 return 1;

         memset(&sa, 0, sizeof(sa));
         sa.sin_family = AF_INET;
         sa.sin_addr.s_addr = inet_addr("127.0.0.1");
         sa.sin_port = htons(11111);

         sendto(fd, NULL, 1, 0, (struct sockaddr *)&sa, sizeof(sa));

         return 0;
 }

We get -ENOMEM:

 $ strace -e sendto ./demo
 sendto(3, NULL, 1, 0, {sa_family=AF_INET, sin_port=htons(11111), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 ENOMEM (Cannot allocate memory)

Propagate the error code from sctp_user_addto_chunk(), so that we will
tell user space what actually went wrong:

 $ strace -e sendto ./demo
 sendto(3, NULL, 1, 0, {sa_family=AF_INET, sin_port=htons(11111), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EFAULT (Bad address)

Noticed while running Trinity (the syscall fuzzer).
Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/chunk.c

@@ -183,7 +183,7 @@ struct sctp_datamsg *sctp_datamsg_from_user(struct sctp_association *asoc,
 
 	msg = sctp_datamsg_new(GFP_KERNEL);
 	if (!msg)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	/* Note: Calculate this outside of the loop, so that all fragments
 	 * have the same expiration.
@@ -280,8 +280,11 @@ struct sctp_datamsg *sctp_datamsg_from_user(struct sctp_association *asoc,
 
 		chunk = sctp_make_datafrag_empty(asoc, sinfo, len, frag, 0);
 
-		if (!chunk)
+		if (!chunk) {
+			err = -ENOMEM;
 			goto errout;
+		}
+
 		err = sctp_user_addto_chunk(chunk, offset, len, msgh->msg_iov);
 		if (err < 0)
 			goto errout_chunk_free;
@@ -315,8 +318,10 @@ struct sctp_datamsg *sctp_datamsg_from_user(struct sctp_association *asoc,
 
 		chunk = sctp_make_datafrag_empty(asoc, sinfo, over, frag, 0);
 
-		if (!chunk)
+		if (!chunk) {
+			err = -ENOMEM;
 			goto errout;
+		}
 
 		err = sctp_user_addto_chunk(chunk, offset, over,msgh->msg_iov);
 
@@ -342,7 +347,7 @@ struct sctp_datamsg *sctp_datamsg_from_user(struct sctp_association *asoc,
 		sctp_chunk_free(chunk);
 	}
 	sctp_datamsg_put(msg);
-	return NULL;
+	return ERR_PTR(err);
 }
 
 /* Check whether this message has expired. */

net/sctp/socket.c

@@ -1915,8 +1915,8 @@ SCTP_STATIC int sctp_sendmsg(struct kiocb *iocb, struct sock *sk,
 
 	/* Break the message into multiple chunks of maximum size. */
 	datamsg = sctp_datamsg_from_user(asoc, sinfo, msg, msg_len);
-	if (!datamsg) {
-		err = -ENOMEM;
+	if (IS_ERR(datamsg)) {
+		err = PTR_ERR(datamsg);
 		goto out_free;
 	}