[linux-pam] Introduce pam_modutil_sanitize_helper_fds

[Dmitry V. Levin]

commit b0ec5d1e472a0cd74972bfe9575dcf6a3d0cad1c
Author: Dmitry V. Levin <ldv(a)altlinux.org&gt;
Date:   Fri Jan 24 15:32:08 2014 +0000

    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.

 libpam/Makefile.am                    |    2 +-
 libpam/include/security/pam_modutil.h |   13 +++
 libpam/libpam.map                     |    5 +
 libpam/pam_modutil_sanitize.c         |  175 +++++++++++++++++++++++++++++++++
 modules/pam_exec/pam_exec.c           |   34 ++-----
 modules/pam_mkhomedir/pam_mkhomedir.c |   15 +--
 modules/pam_unix/pam_unix_acct.c      |   23 ++---
 modules/pam_unix/pam_unix_passwd.c    |   20 ++--
 modules/pam_unix/support.c            |   20 ++--
 modules/pam_unix/support.h            |    2 -
 modules/pam_xauth/pam_xauth.c         |   26 +++--
 11 files changed, 251 insertions(+), 84 deletions(-)
---
diff --git a/libpam/Makefile.am b/libpam/Makefile.am
index 417ca77..685a797 100644
--- a/libpam/Makefile.am
+++ b/libpam/Makefile.am
@@ -43,4 +43,4 @@ libpam_la_SOURCES = pam_account.c pam_auth.c pam_data.c pam_delay.c \
 	pam_modutil_cleanup.c pam_modutil_getpwnam.c pam_modutil_ioloop.c \
 	pam_modutil_getgrgid.c pam_modutil_getpwuid.c pam_modutil_getgrnam.c \
 	pam_modutil_getspnam.c pam_modutil_getlogin.c pam_modutil_ingroup.c \
-	pam_modutil_priv.c
+	pam_modutil_priv.c pam_modutil_sanitize.c
diff --git a/libpam/include/security/pam_modutil.h
b/libpam/include/security/pam_modutil.h
index 8087ba1..4ce8c55 100644
--- a/libpam/include/security/pam_modutil.h
+++ b/libpam/include/security/pam_modutil.h
@@ -129,6 +129,19 @@ extern int PAM_NONNULL((1,2))
 pam_modutil_regain_priv(pam_handle_t *pamh,
 		      struct pam_modutil_privs *p);
 
+enum pam_modutil_redirect_fd {
+	PAM_MODUTIL_IGNORE_FD,	/* do not redirect */
+	PAM_MODUTIL_PIPE_FD,	/* redirect to a pipe */
+	PAM_MODUTIL_NULL_FD,	/* redirect to /dev/null */
+};
+
+/* redirect standard descriptors, close all other descriptors. */
+extern int PAM_NONNULL((1))
+pam_modutil_sanitize_helper_fds(pam_handle_t *pamh,
+				enum pam_modutil_redirect_fd redirect_stdin,
+				enum pam_modutil_redirect_fd redirect_stdout,
+				enum pam_modutil_redirect_fd redirect_stderr);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/libpam/libpam.map b/libpam/libpam.map
index b0885d6..d6835b4 100644
--- a/libpam/libpam.map
+++ b/libpam/libpam.map
@@ -67,3 +67,8 @@ LIBPAM_MODUTIL_1.1.3 {
     pam_modutil_drop_priv;
     pam_modutil_regain_priv;
 } LIBPAM_MODUTIL_1.1;
+
+LIBPAM_MODUTIL_1.1.9 {
+  global:
+    pam_modutil_sanitize_helper_fds;
+} LIBPAM_MODUTIL_1.1.3;
diff --git a/libpam/pam_modutil_sanitize.c b/libpam/pam_modutil_sanitize.c
new file mode 100644
index 0000000..65f85d0
--- /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;
+}
diff --git a/modules/pam_exec/pam_exec.c b/modules/pam_exec/pam_exec.c
index b56e4b2..12c4444 100644
--- a/modules/pam_exec/pam_exec.c
+++ b/modules/pam_exec/pam_exec.c
@@ -302,6 +302,10 @@ call_exec (const char *pam_type, pam_handle_t *pamh,
       char **envlist, **tmp;
       int envlen, nitems;
       char *envstr;
+      enum pam_modutil_redirect_fd redirect_stdin =
+	      expose_authtok ? PAM_MODUTIL_IGNORE_FD : PAM_MODUTIL_PIPE_FD;
+      enum pam_modutil_redirect_fd redirect_stdout =
+	      (use_stdout || logfile) ? PAM_MODUTIL_IGNORE_FD : PAM_MODUTIL_NULL_FD;
 
       /* First, move all the pipes off of stdin, stdout, and stderr, to ensure
        * that calls to dup2 won't close them. */
@@ -330,18 +334,6 @@ call_exec (const char *pam_type, pam_handle_t *pamh,
 	      _exit (err);
 	    }
 	}
-      else
-	{
-	  close (STDIN_FILENO);
-
-	  /* New stdin.  */
-	  if ((i = open ("/dev/null", O_RDWR)) < 0)
-	    {
-	      int err = errno;
-	      pam_syslog (pamh, LOG_ERR, "open of /dev/null failed: %m");
-	      _exit (err);
-	    }
-	}
 
       /* Set up stdout. */
 
@@ -374,26 +366,18 @@ call_exec (const char *pam_type, pam_handle_t *pamh,
 	      free (buffer);
 	    }
 	}
-      else
-	{
-	  close (STDOUT_FILENO);
-	  if ((i = open ("/dev/null", O_RDWR)) < 0)
-	    {
-	      int err = errno;
-	      pam_syslog (pamh, LOG_ERR, "open of /dev/null failed: %m");
-	      _exit (err);
-	    }
-	}
 
-      if (dup2 (STDOUT_FILENO, STDERR_FILENO) == -1)
+      if ((use_stdout || logfile) &&
+	  dup2 (STDOUT_FILENO, STDERR_FILENO) == -1)
 	{
 	  int err = errno;
 	  pam_syslog (pamh, LOG_ERR, "dup2 failed: %m");
 	  _exit (err);
 	}
 
-      for (i = 3; i < sysconf (_SC_OPEN_MAX); i++)
-	close (i);
+      if (pam_modutil_sanitize_helper_fds(pamh, redirect_stdin,
+					  redirect_stdout, redirect_stdout) < 0)
+	_exit(1);
 
       if (call_setuid)
 	if (setuid (geteuid ()) == -1)
diff --git a/modules/pam_mkhomedir/pam_mkhomedir.c
b/modules/pam_mkhomedir/pam_mkhomedir.c
index a867a73..c922089 100644
--- a/modules/pam_mkhomedir/pam_mkhomedir.c
+++ b/modules/pam_mkhomedir/pam_mkhomedir.c
@@ -58,8 +58,6 @@
 #include <security/pam_modutil.h>
 #include <security/pam_ext.h>
 
-#define MAX_FD_NO 10000
-
 /* argument parsing */
 #define MKHOMEDIR_DEBUG      020	/* be verbose about things */
 #define MKHOMEDIR_QUIET      040	/* keep quiet about things */
@@ -131,18 +129,13 @@ create_homedir (pam_handle_t *pamh, options_t *opt,
    /* fork */
    child = fork();
    if (child == 0) {
-        int i;
-        struct rlimit rlim;
 	static char *envp[] = { NULL };
 	const char *args[] = { NULL, NULL, NULL, NULL, NULL };
 
-	if (getrlimit(RLIMIT_NOFILE, &rlim)==0) {
-          if (rlim.rlim_max >= MAX_FD_NO)
-                rlim.rlim_max = MAX_FD_NO;
-	  for (i=0; i < (int)rlim.rlim_max; i++) {
-		close(i);
-	  }
-	}
+	if (pam_modutil_sanitize_helper_fds(pamh, PAM_MODUTIL_PIPE_FD,
+					    PAM_MODUTIL_PIPE_FD,
+					    PAM_MODUTIL_PIPE_FD) < 0)
+		_exit(PAM_SYSTEM_ERR);
 
 	/* exec the mkhomedir helper */
 	args[0] = MKHOMEDIR_HELPER;
diff --git a/modules/pam_unix/pam_unix_acct.c b/modules/pam_unix/pam_unix_acct.c
index dc505e7..2799845 100644
--- a/modules/pam_unix/pam_unix_acct.c
+++ b/modules/pam_unix/pam_unix_acct.c
@@ -98,24 +98,21 @@ int _unix_run_verify_binary(pam_handle_t *pamh, unsigned int ctrl,
   /* fork */
   child = fork();
   if (child == 0) {
-    int i=0;
-    struct rlimit rlim;
     static char *envp[] = { NULL };
     const char *args[] = { NULL, NULL, NULL, NULL };
 
-    /* reopen stdout as pipe */
-    dup2(fds[1], STDOUT_FILENO);
-
     /* XXX - should really tidy up PAM here too */
 
-    if (getrlimit(RLIMIT_NOFILE,&rlim)==0) {
-      if (rlim.rlim_max >= MAX_FD_NO)
-        rlim.rlim_max = MAX_FD_NO;
-      for (i=0; i < (int)rlim.rlim_max; i++) {
-	if (i != STDOUT_FILENO) {
-	  close(i);
-	}
-      }
+    /* reopen stdout as pipe */
+    if (dup2(fds[1], STDOUT_FILENO) != STDOUT_FILENO) {
+      pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", "stdout");
+      _exit(PAM_AUTHINFO_UNAVAIL);
+    }
+
+    if (pam_modutil_sanitize_helper_fds(pamh, PAM_MODUTIL_PIPE_FD,
+					PAM_MODUTIL_IGNORE_FD,
+					PAM_MODUTIL_PIPE_FD) < 0) {
+      _exit(PAM_AUTHINFO_UNAVAIL);
     }
 
     if (geteuid() == 0) {
diff --git a/modules/pam_unix/pam_unix_passwd.c b/modules/pam_unix/pam_unix_passwd.c
index 5f3a3db..606071e 100644
--- a/modules/pam_unix/pam_unix_passwd.c
+++ b/modules/pam_unix/pam_unix_passwd.c
@@ -201,8 +201,6 @@ static int _unix_run_update_binary(pam_handle_t *pamh, unsigned int
ctrl, const
     /* fork */
     child = fork();
     if (child == 0) {
-        int i=0;
-        struct rlimit rlim;
 	static char *envp[] = { NULL };
 	const char *args[] = { NULL, NULL, NULL, NULL, NULL, NULL };
         char buffer[16];
@@ -210,15 +208,15 @@ static int _unix_run_update_binary(pam_handle_t *pamh, unsigned int
ctrl, const
 	/* XXX - should really tidy up PAM here too */
 
 	/* reopen stdin as pipe */
-	dup2(fds[0], STDIN_FILENO);
-
-	if (getrlimit(RLIMIT_NOFILE,&rlim)==0) {
-	  if (rlim.rlim_max >= MAX_FD_NO)
-	    rlim.rlim_max = MAX_FD_NO;
-	  for (i=0; i < (int)rlim.rlim_max; i++) {
-	    if (i != STDIN_FILENO)
-		close(i);
-	  }
+	if (dup2(fds[0], STDIN_FILENO) != STDIN_FILENO) {
+		pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", "stdin");
+		_exit(PAM_AUTHINFO_UNAVAIL);
+	}
+
+	if (pam_modutil_sanitize_helper_fds(pamh, PAM_MODUTIL_IGNORE_FD,
+					    PAM_MODUTIL_PIPE_FD,
+					    PAM_MODUTIL_PIPE_FD) < 0) {
+		_exit(PAM_AUTHINFO_UNAVAIL);
 	}
 
 	/* exec binary helper */
diff --git a/modules/pam_unix/support.c b/modules/pam_unix/support.c
index 3a849c8..fdb45c2 100644
--- a/modules/pam_unix/support.c
+++ b/modules/pam_unix/support.c
@@ -564,23 +564,21 @@ static int _unix_run_helper_binary(pam_handle_t *pamh, const char
*passwd,
     /* fork */
     child = fork();
     if (child == 0) {
-        int i=0;
-        struct rlimit rlim;
 	static char *envp[] = { NULL };
 	const char *args[] = { NULL, NULL, NULL, NULL };
 
 	/* XXX - should really tidy up PAM here too */
 
 	/* reopen stdin as pipe */
-	dup2(fds[0], STDIN_FILENO);
-
-	if (getrlimit(RLIMIT_NOFILE,&rlim)==0) {
-          if (rlim.rlim_max >= MAX_FD_NO)
-                rlim.rlim_max = MAX_FD_NO;
-	  for (i=0; i < (int)rlim.rlim_max; i++) {
-		if (i != STDIN_FILENO)
-		  close(i);
-	  }
+	if (dup2(fds[0], STDIN_FILENO) != STDIN_FILENO) {
+		pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", "stdin");
+		_exit(PAM_AUTHINFO_UNAVAIL);
+	}
+
+	if (pam_modutil_sanitize_helper_fds(pamh, PAM_MODUTIL_IGNORE_FD,
+					    PAM_MODUTIL_PIPE_FD,
+					    PAM_MODUTIL_PIPE_FD) < 0) {
+		_exit(PAM_AUTHINFO_UNAVAIL);
 	}
 
 	if (geteuid() == 0) {
diff --git a/modules/pam_unix/support.h b/modules/pam_unix/support.h
index 6f5b2eb..cd6ddb7 100644
--- a/modules/pam_unix/support.h
+++ b/modules/pam_unix/support.h
@@ -141,8 +141,6 @@ static const UNIX_Ctrls unix_args[UNIX_CTRLS_] =
 
 #define UNIX_DEFAULTS  (unix_args[UNIX__NONULL].flag)
 
-#define MAX_FD_NO 2000000
-
 /* use this to free strings. ESPECIALLY password strings */
 
 #define _pam_delete(xx)		\
diff --git a/modules/pam_xauth/pam_xauth.c b/modules/pam_xauth/pam_xauth.c
index c7ce55a..2be4351 100644
--- a/modules/pam_xauth/pam_xauth.c
+++ b/modules/pam_xauth/pam_xauth.c
@@ -128,7 +128,6 @@ run_coprocess(pam_handle_t *pamh, const char *input, char **output,
 		/* We're the child. */
 		size_t j;
 		const char *args[10];
-		int maxopened;
 		/* Drop privileges. */
 		if (setgid(gid) == -1)
 		  {
@@ -150,19 +149,26 @@ run_coprocess(pam_handle_t *pamh, const char *input, char **output,
 				(unsigned long) geteuid ());
 		    _exit (err);
 		  }
-		/* Initialize the argument list. */
-		memset(args, 0, sizeof(args));
 		/* Set the pipe descriptors up as stdin and stdout, and close
 		 * everything else, including the original values for the
 		 * descriptors. */
-		dup2(ipipe[0], STDIN_FILENO);
-		dup2(opipe[1], STDOUT_FILENO);
-		maxopened = (int)sysconf(_SC_OPEN_MAX);
-		for (i = 0; i < maxopened; i++) {
-			if ((i != STDIN_FILENO) && (i != STDOUT_FILENO)) {
-				close(i);
-			}
+		if (dup2(ipipe[0], STDIN_FILENO) != STDIN_FILENO) {
+		    int err = errno;
+		    pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", "stdin");
+		    _exit(err);
 		}
+		if (dup2(opipe[1], STDOUT_FILENO) != STDOUT_FILENO) {
+		    int err = errno;
+		    pam_syslog(pamh, LOG_ERR, "dup2 of %s failed: %m", "stdout");
+		    _exit(err);
+		}
+		if (pam_modutil_sanitize_helper_fds(pamh, PAM_MODUTIL_IGNORE_FD,
+						    PAM_MODUTIL_IGNORE_FD,
+						    PAM_MODUTIL_NULL_FD) < 0) {
+		    _exit(1);
+		}
+		/* Initialize the argument list. */
+		memset(args, 0, sizeof(args));
 		/* Convert the varargs list into a regular array of strings. */
 		va_start(ap, command);
 		args[0] = command;