Fix portability bugs in use of credentials control messages for peer auth.

Even though our existing code for handling credentials control messages has
been basically unchanged since 2001, it was fundamentally wrong: it did not
ensure proper alignment of the supplied buffer, and it was calculating
buffer sizes and message sizes incorrectly.  This led to failures on
platforms where alignment padding is relevant, for instance FreeBSD on
64-bit platforms, as seen in a recent Debian bug report passed on by
Martin Pitt (http://bugs.debian.org//cgi-bin/bugreport.cgi?bug=612888).

Rewrite to do the message-whacking using the macros specified in RFC 2292,
following a suggestion from Theo de Raadt in that thread.  Tested by me
on Debian/kFreeBSD-amd64; since OpenBSD and NetBSD document the identical
CMSG API, it should work there too.

Back-patch to all supported branches.

Author: tglsfdc

src/backend/libpq/hba.c

@@ -1453,7 +1453,7 @@ static bool
 ident_unix(int sock, char *ident_user)
 {
 #if defined(HAVE_GETPEEREID)
-	/* OpenBSD style:  */
+	/* OpenBSD (also Mac OS X) style: use getpeereid() */
 	uid_t		uid;
 	gid_t		gid;
 	struct passwd *pass;
@@ -1512,9 +1512,7 @@ ident_unix(int sock, char *ident_user)
 
 	return true;
 #elif defined(HAVE_STRUCT_CMSGCRED) || defined(HAVE_STRUCT_FCRED) || (defined(HAVE_STRUCT_SOCKCRED) && defined(LOCAL_CREDS))
-	struct msghdr msg;
-
-/* Credentials structure */
+	/* Assorted BSDen: use a credentials control message */
 #if defined(HAVE_STRUCT_CMSGCRED)
 	typedef struct cmsgcred Cred;
 
@@ -1528,43 +1526,55 @@ ident_unix(int sock, char *ident_user)
 
 #define cruid sc_uid
 #endif
-	Cred	   *cred;
-
-	/* Compute size without padding */
-	char		cmsgmem[ALIGN(sizeof(struct cmsghdr)) + ALIGN(sizeof(Cred))];	/* for NetBSD */
-
-	/* Point to start of first structure */
-	struct cmsghdr *cmsg = (struct cmsghdr *) cmsgmem;
 
+	struct msghdr msg;
+	struct cmsghdr *cmsg;
+	union
+	{
+		struct cmsghdr	hdr;
+		unsigned char	buf[CMSG_SPACE(sizeof(Cred))];
+	} cmsgbuf;
 	struct iovec iov;
 	char		buf;
+	Cred	   *cred;
 	struct passwd *pw;
 
-	memset(&msg, 0, sizeof(msg));
-	msg.msg_iov = &iov;
-	msg.msg_iovlen = 1;
-	msg.msg_control = (char *) cmsg;
-	msg.msg_controllen = sizeof(cmsgmem);
-	memset(cmsg, 0, sizeof(cmsgmem));
-
 	/*
-	 * The one character which is received here is not meaningful; its
-	 * purposes is only to make sure that recvmsg() blocks long enough for the
-	 * other side to send its credentials.
+	 * The one character that is received here is not meaningful; its purpose
+	 * is only to make sure that recvmsg() blocks long enough for the other
+	 * side to send its credentials.
 	 */
 	iov.iov_base = &buf;
 	iov.iov_len = 1;
 
-	if (recvmsg(sock, &msg, 0) < 0 ||
-		cmsg->cmsg_len < sizeof(cmsgmem) ||
-		cmsg->cmsg_type != SCM_CREDS)
+	memset(&msg, 0, sizeof(msg));
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = &cmsgbuf.buf;
+	msg.msg_controllen = sizeof(cmsgbuf.buf);
+	memset(&cmsgbuf, 0, sizeof(cmsgbuf));
+
+	if (recvmsg(sock, &msg, 0) < 0)
 	{
 		ereport(LOG,
 				(errcode_for_socket_access(),
 				 errmsg("could not get peer credentials: %m")));
 		return false;
 	}
 
+	cmsg = CMSG_FIRSTHDR(&msg);
+	if (msg.msg_flags & (MSG_TRUNC | MSG_CTRUNC) ||
+		cmsg == NULL ||
+		cmsg->cmsg_len < CMSG_LEN(sizeof(Cred)) ||
+		cmsg->cmsg_level != SOL_SOCKET ||
+		cmsg->cmsg_type != SCM_CREDS)
+	{
+		ereport(LOG,
+				(errcode(ERRCODE_PROTOCOL_VIOLATION),
+				 errmsg("could not get peer credentials: incorrect control message")));
+		return false;
+	}
+
 	cred = (Cred *) CMSG_DATA(cmsg);
 
 	pw = getpwuid(cred->cruid);

src/interfaces/libpq/fe-auth.c

@@ -709,11 +709,12 @@ pg_local_sendauth(PGconn *conn)
 	struct msghdr msg;
 
 #ifdef HAVE_STRUCT_CMSGCRED
-	/* Prevent padding */
-	char		cmsgmem[sizeof(struct cmsghdr) + sizeof(struct cmsgcred)];
-
-	/* Point to start of first structure */
-	struct cmsghdr *cmsg = (struct cmsghdr *) cmsgmem;
+	struct cmsghdr *cmsg;
+	union
+	{
+		struct cmsghdr	hdr;
+		unsigned char	buf[CMSG_SPACE(sizeof(struct cmsgcred))];
+	} cmsgbuf;
 #endif
 
 	/*
@@ -729,11 +730,12 @@ pg_local_sendauth(PGconn *conn)
 	msg.msg_iovlen = 1;
 
 #ifdef HAVE_STRUCT_CMSGCRED
-	/* Create control header, FreeBSD */
-	msg.msg_control = cmsg;
-	msg.msg_controllen = sizeof(cmsgmem);
-	memset(cmsg, 0, sizeof(cmsgmem));
-	cmsg->cmsg_len = sizeof(cmsgmem);
+	/* FreeBSD needs us to set up a message that will be filled in by kernel */
+	memset(&cmsgbuf, 0, sizeof(cmsgbuf));
+	msg.msg_control = &cmsgbuf.buf;
+	msg.msg_controllen = sizeof(cmsgbuf.buf);
+	cmsg = CMSG_FIRSTHDR(&msg);
+	cmsg->cmsg_len = CMSG_LEN(sizeof(struct cmsgcred));
 	cmsg->cmsg_level = SOL_SOCKET;
 	cmsg->cmsg_type = SCM_CREDS;
 #endif