lib/, src/: Add checks for fd omission

Adding function check_fds to new file fd.c. The function check_fds
should be called in every setuid/setgid program.

Co-developed-by: Alejandro Colomar <alx@kernel.org>
Cherry-picked-from: d2f2c1877a ("Adding checks for fd omission")
Link: <https://github.com/shadow-maint/shadow/pull/964>
Link: <https://inbox.sourceware.org/libc-alpha/ZeyujhVRsDTUNUtw@debian/T/>
[alx: It seems we shouldn't need this, as libc does it for us.  But it ]
[     shouldn't hurt either.  Let's be paranoic.                       ]
Cc: <Guillem Jover <guillem@hadrons.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: "Skyler Ferrante (RIT Student)" <sjf5462@rit.edu>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Christian Brauner <christian@brauner.io>
Cc: Rich Felker <dalias@libc.org>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Cc: Thorsten Glaser <tg@mirbsd.de>
Cc: NRK <nrk@disroot.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: enh <enh@google.com>
Cc: Laurent Bercot <ska-dietlibc@skarnet.org>
Cc: Gabriel Ravier <gabravier@gmail.com>
Cc: Zack Weinberg <zack@owlfolio.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/Makefile.am

@@ -53,6 +53,7 @@ libshadow_la_SOURCES = \
 	faillog.h \
 	failure.c \
 	failure.h \
+	fd.c \
 	fields.c \
 	find_new_gid.c \
 	find_new_uid.c \

lib/fd.c

@@ -0,0 +1,41 @@
+// SPDX-FileCopyrightText: 2024, Skyler Ferrante <sjf5462@rit.edu>
+// SPDX-License-Identifier: BSD-3-Clause
+
+/**
+ * To protect against file descriptor omission attacks, we open the std file
+ * descriptors with /dev/null if they are not already open. Code is based on
+ * fix_fds from sudo.c.
+ */
+
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "prototypes.h"
+
+static void check_fd(int fd);
+
+void
+check_fds(void)
+{
+	/**
+	 * Make sure stdin, stdout, stderr are open
+	 * If they are closed, set them to /dev/null
+	 */
+	check_fd(STDIN_FILENO);
+	check_fd(STDOUT_FILENO);
+	check_fd(STDERR_FILENO);
+}
+
+static void
+check_fd(int fd)
+{
+	int  devnull;
+
+	if (fcntl(fd, F_GETFL, 0) != -1)
+		return;
+
+	devnull = open("/dev/null", O_RDWR);
+	if (devnull != fd)
+		abort();
+}

lib/prototypes.h

@@ -127,6 +127,9 @@ extern void initenv (void);
 extern void set_env (int, char *const *);
 extern void sanitize_env (void);
 
+/* fd.c */
+extern void check_fds (void);
+
 /* fields.c */
 extern void change_field (char *, size_t, const char *);
 extern int valid_field (const char *, const char *);