Introduce pam_modutil_sanitize_helper_fds

This change introduces pam_modutil_sanitize_helper_fds - a new function
that redirects standard descriptors and closes all other descriptors.

pam_modutil_sanitize_helper_fds supports three types of input and output
redirection:
- PAM_MODUTIL_IGNORE_FD: do not redirect at all.
- PAM_MODUTIL_PIPE_FD: redirect to a pipe.  For stdin, it is implemented
  by creating a pipe, closing its write end, and redirecting stdin to
  its read end.  Likewise, for stdout/stderr it is implemented by
  creating a pipe, closing its read end, and redirecting to its write
  end.  Unlike stdin redirection, stdout/stderr redirection to a pipe
  has a side effect that a process writing to such descriptor should be
  prepared to handle SIGPIPE appropriately.
- PAM_MODUTIL_NULL_FD: redirect to /dev/null.  For stdin, it is
  implemented via PAM_MODUTIL_PIPE_FD because there is no functional
  difference.  For stdout/stderr, it is classic redirection to
  /dev/null.

PAM_MODUTIL_PIPE_FD is usually more suitable due to linux kernel
security restrictions, but when the helper process might be writing to
the corresponding descriptor and termination of the helper process by
SIGPIPE is not desirable, one should choose PAM_MODUTIL_NULL_FD.

* libpam/pam_modutil_sanitize.c: New file.
* libpam/Makefile.am (libpam_la_SOURCES): Add it.
* libpam/include/security/pam_modutil.h (pam_modutil_redirect_fd,
pam_modutil_sanitize_helper_fds): New declarations.
* libpam/libpam.map (LIBPAM_MODUTIL_1.1.9): New interface.
* modules/pam_exec/pam_exec.c (call_exec): Use
pam_modutil_sanitize_helper_fds.
* modules/pam_mkhomedir/pam_mkhomedir.c (create_homedir): Likewise.
* modules/pam_unix/pam_unix_acct.c (_unix_run_verify_binary): Likewise.
* modules/pam_unix/pam_unix_passwd.c (_unix_run_update_binary):
Likewise.
* modules/pam_unix/support.c (_unix_run_helper_binary): Likewise.
* modules/pam_xauth/pam_xauth.c (run_coprocess): Likewise.
* modules/pam_unix/support.h (MAX_FD_NO): Remove.

1 files changed, 175 insertions, 0 deletions

diff --git a/libpam/pam_modutil_sanitize.c b/libpam/pam_modutil_sanitize.c
new file mode 100644
index 00000000..65f85d01
--- /dev/null
+++ b/libpam/pam_modutil_sanitize.c
@@ -0,0 +1,175 @@
+/*
+ * This file implements the following functions:
+ *   pam_modutil_sanitize_helper_fds:
+ *     redirects standard descriptors, closes all other descriptors.
+ */
+
+#include "pam_modutil_private.h"
+#include <security/pam_ext.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <syslog.h>
+#include <sys/resource.h>
+
+/*
+ * Creates a pipe, closes its write end, redirects fd to its read end.
+ * Returns fd on success, -1 otherwise.
+ */
+static int
+redirect_in_pipe(pam_handle_t *pamh, int fd, const char *name)
+{
+	int in[2];
+
+	if (pipe(in) < 0) {
+		pam_syslog(pamh, LOG_ERR, "Could not create pipe: %m");
+		return -1;
+	}
+
+	close(in[1]);
+
+	if (in[0] == fd)
+		return fd;
+
+	if (dup2(in[0], fd) != fd) {
+		pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", name);
+		fd = -1;
+	}
+
+	close(in[0]);
+	return fd;
+}
+
+/*
+ * Creates a pipe, closes its read end, redirects fd to its write end.
+ * Returns fd on success, -1 otherwise.
+ */
+static int
+redirect_out_pipe(pam_handle_t *pamh, int fd, const char *name)
+{
+	int out[2];
+
+	if (pipe(out) < 0) {
+		pam_syslog(pamh, LOG_ERR, "Could not create pipe: %m");
+		return -1;
+	}
+
+	close(out[0]);
+
+	if (out[1] == fd)
+		return fd;
+
+	if (dup2(out[1], fd) != fd) {
+		pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", name);
+		fd = -1;
+	}
+
+	close(out[1]);
+	return fd;
+}
+
+/*
+ * Opens /dev/null for writing, redirects fd there.
+ * Returns fd on success, -1 otherwise.
+ */
+static int
+redirect_out_null(pam_handle_t *pamh, int fd, const char *name)
+{
+	int null = open("/dev/null", O_WRONLY);
+
+	if (null < 0) {
+		pam_syslog(pamh, LOG_ERR, "open of %s failed: %m", "/dev/null");
+		return -1;
+	}
+
+	if (null == fd)
+		return fd;
+
+	if (dup2(null, fd) != fd) {
+		pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", name);
+		fd = -1;
+	}
+
+	close(null);
+	return fd;
+}
+
+static int
+redirect_out(pam_handle_t *pamh, enum pam_modutil_redirect_fd mode,
+	     int fd, const char *name)
+{
+	switch (mode) {
+		case PAM_MODUTIL_PIPE_FD:
+			if (redirect_out_pipe(pamh, fd, name) < 0)
+				return -1;
+			break;
+		case PAM_MODUTIL_NULL_FD:
+			if (redirect_out_null(pamh, fd, name) < 0)
+				return -1;
+			break;
+		case PAM_MODUTIL_IGNORE_FD:
+			break;
+	}
+	return fd;
+}
+
+/* Closes all descriptors after stderr. */
+static void
+close_fds(void)
+{
+	/*
+	 * An arbitrary upper limit for the maximum file descriptor number
+	 * returned by RLIMIT_NOFILE.
+	 */
+	const int MAX_FD_NO = 65535;
+
+	/* The lower limit is the same as for _POSIX_OPEN_MAX. */
+	const int MIN_FD_NO = 20;
+
+	int fd;
+	struct rlimit rlim;
+
+	if (getrlimit(RLIMIT_NOFILE, &rlim) || rlim.rlim_max > MAX_FD_NO)
+		fd = MAX_FD_NO;
+	else if (rlim.rlim_max < MIN_FD_NO)
+		fd = MIN_FD_NO;
+	else
+		fd = rlim.rlim_max - 1;
+
+	for (; fd > STDERR_FILENO; --fd)
+		close(fd);
+}
+
+int
+pam_modutil_sanitize_helper_fds(pam_handle_t *pamh,
+				enum pam_modutil_redirect_fd stdin_mode,
+				enum pam_modutil_redirect_fd stdout_mode,
+				enum pam_modutil_redirect_fd stderr_mode)
+{
+	if (stdin_mode != PAM_MODUTIL_IGNORE_FD &&
+	    redirect_in_pipe(pamh, STDIN_FILENO, "stdin") < 0) {
+		return -1;
+	}
+
+	if (redirect_out(pamh, stdout_mode, STDOUT_FILENO, "stdout") < 0)
+		return -1;
+
+	/*
+	 * If stderr should not be ignored and
+	 * redirect mode for stdout and stderr are the same,
+	 * optimize by redirecting stderr to stdout.
+	 */
+	if (stderr_mode != PAM_MODUTIL_IGNORE_FD &&
+	    stdout_mode == stderr_mode) {
+		if (dup2(STDOUT_FILENO, STDERR_FILENO) != STDERR_FILENO) {
+			pam_syslog(pamh, LOG_ERR,
+				   "dup2 of %s failed: %m", "stderr");
+			return -1;
+		}
+	} else {
+		if (redirect_out(pamh, stderr_mode, STDERR_FILENO, "stderr") < 0)
+			return -1;
+	}
+
+	close_fds();
+	return 0;
+}