Allow SSL server key file to have group read access if owned by root

We used to require the server key file to have permissions 0600 or less
for best security.  But some systems (such as Debian) have certificate
and key files managed by the operating system that can be shared with
other services.  In those cases, the "postgres" user is made a member of
a special group that has access to those files, and the server key file
has permissions 0640.  To accommodate that kind of setup, also allow the
key file to have permissions 0640 but only if owned by root.

From: Christoph Berg <myon@debian.org>
Reviewed-by: NAlvaro Herrera <alvherre@alvh.no-ip.org>

doc/src/sgml/runtime.sgml

@@ -2147,9 +2147,20 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
    the server's data directory, but other names and locations can be specified
    using the configuration parameters <xref linkend="guc-ssl-cert-file">
    and <xref linkend="guc-ssl-key-file">.
+  </para>
+
+  <para>
    On Unix systems, the permissions on <filename>server.key</filename> must
    disallow any access to world or group; achieve this by the command
-   <command>chmod 0600 server.key</command>.
+   <command>chmod 0600 server.key</command>.  Alternatively, the file can be
+   owned by root and have group read access (that is, <literal>0640</literal>
+   permissions).  That setup is intended for installations where certificate
+   and key files are managed by the operating system.  The user under which
+   the <productname>PostgreSQL</productname> server runs should then be made a
+   member of the group that has access to those certificate and key files.
+  </para>
+
+  <para>
    If the private key is protected with a passphrase, the
    server will prompt for the passphrase and will not start until it has
    been entered.

src/backend/libpq/be-secure-openssl.c

@@ -206,8 +206,30 @@ be_tls_init(void)
 					 errmsg("could not access private key file \"%s\": %m",
 							ssl_key_file)));
 
+		if (!S_ISREG(buf.st_mode))
+			ereport(FATAL,
+					(errcode(ERRCODE_CONFIG_FILE_ERROR),
+					 errmsg("private key file \"%s\" is not a regular file",
+							ssl_key_file)));
+
+		/*
+		 * Refuse to load files owned by users other than us or root.
+		 *
+		 * XXX surely we can check this on Windows somehow, too.
+		 */
+#if !defined(WIN32) && !defined(__CYGWIN__)
+		if (buf.st_uid != geteuid() && buf.st_uid != 0)
+			ereport(FATAL,
+					(errcode(ERRCODE_CONFIG_FILE_ERROR),
+					 errmsg("private key file \"%s\" must be owned by the database user or root",
+							ssl_key_file)));
+#endif
+
 		/*
-		 * Require no public access to key file.
+		 * Require no public access to key file. If the file is owned by us,
+		 * require mode 0600 or less. If owned by root, require 0640 or less
+		 * to allow read access through our gid, or a supplementary gid that
+		 * allows to read system-wide certificates.
 		 *
 		 * XXX temporarily suppress check when on Windows, because there may
 		 * not be proper support for Unix-y file permissions.  Need to think
@@ -215,12 +237,13 @@ be_tls_init(void)
 		 * directory permission check in postmaster.c)
 		 */
 #if !defined(WIN32) && !defined(__CYGWIN__)
-		if (!S_ISREG(buf.st_mode) || buf.st_mode & (S_IRWXG | S_IRWXO))
+		if ((buf.st_uid == geteuid() && buf.st_mode & (S_IRWXG | S_IRWXO)) ||
+			(buf.st_uid == 0 && buf.st_mode & (S_IWGRP | S_IXGRP | S_IRWXO)))
 			ereport(FATAL,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
-				  errmsg("private key file \"%s\" has group or world access",
-						 ssl_key_file),
-				   errdetail("Permissions should be u=rw (0600) or less.")));
+					 errmsg("private key file \"%s\" has group or world access",
+							ssl_key_file),
+					 errdetail("File must have permissions u=rw (0600) or less if owned by the database user, or permissions u=rw,g=r (0640) or less if owned by root.")));
 #endif
 
 		if (SSL_CTX_use_PrivateKey_file(SSL_context,