mount.cifs: properly check for mount being in fstab when running setuid root (try#3)

This is the third attempt to clean up the checks when a setuid
mount.cifs is run by an unprivileged user. The main difference in this
patch from the last one is that it fixes a bug where the mount might
have failed if unnecessarily if CIFS_LEGACY_SETUID_CHECK was set.

When mount.cifs is installed setuid root and run as an unprivileged
user, it does some checks to limit how the mount is used. It checks that
the mountpoint is owned by the user doing the mount.

These checks however do not match those that /bin/mount does when it is
called by an unprivileged user. When /bin/mount is called by an
unprivileged user to do a mount, it checks that the mount in question is
in /etc/fstab, that it has the "user" option set, etc.

This means that it's currently not possible to set up user mounts the
standard way (by the admin, in /etc/fstab) and simultaneously protect
from an unprivileged user calling mount.cifs directly to mount a share
on any directory that that user owns.

Fix this by making the checks in mount.cifs match those of /bin/mount
itself. This is a necessary step to make mount.cifs safe to be installed
as a setuid binary, but not sufficient. For that, we'd need to give
mount.cifs a proper security audit.

Since some users may be depending on the legacy behavior, this patch
also adds the ability to build mount.cifs with the older behavior.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
(cherry picked from commit 396eb03109400fe603c57a0a0d4bdc37c7131cf5)

1 files changed, 162 insertions, 40 deletions

diff --git a/source3/client/mount.cifs.c b/source3/client/mount.cifs.c
index 43dc7f618b6..47d88fd39ad 100644
--- a/source3/client/mount.cifs.c
+++ b/source3/client/mount.cifs.c
@@ -39,10 +39,11 @@
 #include <mntent.h>
 #include <fcntl.h>
 #include <limits.h>
+#include <fstab.h>
 #include "mount.h"
 
 #define MOUNT_CIFS_VERSION_MAJOR "1"
-#define MOUNT_CIFS_VERSION_MINOR "12"
+#define MOUNT_CIFS_VERSION_MINOR "13"
 
 #ifndef MOUNT_CIFS_VENDOR_SUFFIX
  #ifdef _SAMBA_BUILD_
@@ -69,6 +70,10 @@
 #define MS_BIND 4096
 #endif
 
+/* private flags - clear these before passing to kernel */
+#define MS_USERS	0x40000000
+#define MS_USER		0x80000000
+
 #define MAX_UNC_LEN 1024
 
 #define CONST_DISCARD(type, ptr)      ((type) ((void *) (ptr)))
@@ -83,6 +88,27 @@
 /* currently maximum length of IPv6 address string */
 #define MAX_ADDRESS_LEN INET6_ADDRSTRLEN
 
+/*
+ * By default, mount.cifs follows the conventions set forth by /bin/mount
+ * for user mounts. That is, it requires that the mount be listed in
+ * /etc/fstab with the "user" option when run as an unprivileged user and
+ * mount.cifs is setuid root.
+ *
+ * Older versions of mount.cifs however were "looser" in this regard. When
+ * made setuid root, a user could run mount.cifs directly and mount any share
+ * on a directory owned by that user.
+ *
+ * The legacy behavior is now disabled by default. To reenable it, set the
+ * following #define to true.
+ */
+#define CIFS_LEGACY_SETUID_CHECK 0
+
+/*
+ * When an unprivileged user runs a setuid mount.cifs, we set certain mount
+ * flags by default. These defaults can be changed here.
+ */
+#define CIFS_SETUID_FLAGS (MS_NOSUID|MS_NODEV)
+
 const char *thisprogram;
 int verboseflag = 0;
 int fakemnt = 0;
@@ -142,6 +168,99 @@ static size_t strlcat(char *d, const char *s, size_t bufsize)
 }
 #endif
 
+/*
+ * If an unprivileged user is doing the mounting then we need to ensure
+ * that the entry is in /etc/fstab.
+ */
+static int
+check_mountpoint(const char *progname, char *mountpoint)
+{
+	int err;
+	struct stat statbuf;
+
+	/* does mountpoint exist and is it a directory? */
+	err = stat(mountpoint, &statbuf);
+	if (err) {
+		fprintf(stderr, "%s: failed to stat %s: %s\n", progname,
+				mountpoint, strerror(errno));
+		return EX_USAGE;
+	}
+
+	if (!S_ISDIR(statbuf.st_mode)) {
+		fprintf(stderr, "%s: %s is not a directory!", progname,
+				mountpoint);
+		return EX_USAGE;
+	}
+
+#if CIFS_LEGACY_SETUID_CHECK
+	/* do extra checks on mountpoint for legacy setuid behavior */
+	if (!getuid() || geteuid())
+		return 0;
+
+	if (statbuf.st_uid != getuid()) {
+		fprintf(stderr, "%s: %s is not owned by user\n", progname,
+			mountpoint);
+		return EX_USAGE;
+	}
+
+	if ((statbuf.st_mode & S_IRWXU) != S_IRWXU) {
+		fprintf(stderr, "%s: invalid permissions on %s\n", progname,
+			mountpoint);
+		return EX_USAGE;
+	}
+#endif /* CIFS_LEGACY_SETUID_CHECK */
+
+	return 0;
+}
+
+#if CIFS_LEGACY_SETUID_CHECK
+static int
+check_fstab(const char *progname, char *mountpoint, char *devname,
+	    char **options)
+{
+	return 0;
+}
+#else /* CIFS_LEGACY_SETUID_CHECK */
+static int
+check_fstab(const char *progname, char *mountpoint, char *devname,
+	    char **options)
+{
+	FILE *fstab;
+	struct mntent *mnt;
+
+	/* make sure this mount is listed in /etc/fstab */
+	fstab = setmntent(_PATH_FSTAB, "r");
+	if (!fstab) {
+		fprintf(stderr, "Couldn't open %s for reading!\n",
+				_PATH_FSTAB);
+		return EX_FILEIO;
+	}
+
+	while((mnt = getmntent(fstab))) {
+		if (!strcmp(mountpoint, mnt->mnt_dir))
+			break;
+	}
+	endmntent(fstab);
+
+	if (mnt == NULL || strcmp(mnt->mnt_fsname, devname)) {
+		fprintf(stderr, "%s: permission denied: no match for "
+				"%s found in %s\n", progname, mountpoint,
+				_PATH_FSTAB);
+		return EX_USAGE;
+	}
+
+	/*
+	 * 'mount' munges the options from fstab before passing them
+	 * to us. It is non-trivial to test that we have the correct
+	 * set of options. We don't want to trust what the user
+	 * gave us, so just take whatever is in /etc/fstab.
+	 */
+	free(*options);
+	*options = strdup(mnt->mnt_opts);
+	return 0;
+}
+#endif /* CIFS_LEGACY_SETUID_CHECK */
+
 /* BB finish BB
 
         cifs_umount
@@ -373,7 +492,7 @@ static int get_password_from_file(int file_descript, char * filename)
 	return rc;
 }
 
-static int parse_options(char ** optionsp, int * filesys_flags)
+static int parse_options(char ** optionsp, unsigned long * filesys_flags)
 {
 	const char * data;
 	char * percent_char = NULL;
@@ -423,6 +542,7 @@ static int parse_options(char ** optionsp, int * filesys_flags)
 
 		if (strncmp(data, "users",5) == 0) {
 			if(!value || !*value) {
+				*filesys_flags |= MS_USERS;
 				goto nocopy;
 			}
 		} else if (strncmp(data, "user_xattr",10) == 0) {
@@ -431,10 +551,7 @@ static int parse_options(char ** optionsp, int * filesys_flags)
 
 			if (!value || !*value) {
 				if(data[4] == '\0') {
-					if(verboseflag)
-						printf("\nskipping empty user mount parameter\n");
-					/* remove the parm since it would otherwise be confusing
-					to the kernel code which would think it was a real username */
+					*filesys_flags |= MS_USER;
 					goto nocopy;
 				} else {
 					printf("username specified with no parameter\n");
@@ -1046,7 +1163,7 @@ static void print_cifs_mount_version(void)
 int main(int argc, char ** argv)
 {
 	int c;
-	int flags = MS_MANDLOCK; /* no need to set legacy MS_MGC_VAL */
+	unsigned long flags = MS_MANDLOCK;
 	char * orgoptions = NULL;
 	char * share_name = NULL;
 	const char * ipaddr = NULL;
@@ -1069,7 +1186,6 @@ int main(int argc, char ** argv)
 	size_t current_len;
 	int retry = 0; /* set when we have to retry mount with uppercase */
 	struct addrinfo *addrhead = NULL, *addr;
-	struct stat statbuf;
 	struct utsname sysinfo;
 	struct mntent mountent;
 	struct sockaddr_in *addr4;
@@ -1127,8 +1243,8 @@ int main(int argc, char ** argv)
 		exit(EX_USAGE);
 	}
 
-	/* add sharename in opts string as unc= parm */
 
+	/* add sharename in opts string as unc= parm */
 	while ((c = getopt_long (argc, argv, "afFhilL:no:O:rsSU:vVwt:",
 			 longopts, NULL)) != -1) {
 		switch (c) {
@@ -1266,6 +1382,22 @@ int main(int argc, char ** argv)
 		exit(EX_USAGE);
 	}
 
+	/* make sure mountpoint is legit */
+	rc = check_mountpoint(thisprogram, mountpoint);
+	if (rc)
+		goto mount_exit;
+
+	/* sanity check for unprivileged mounts */
+	if (getuid()) {
+		rc = check_fstab(thisprogram, mountpoint, dev_name,
+				 &orgoptions);
+		if (rc)
+			goto mount_exit;
+
+		/* enable any default user mount flags */
+		flags |= CIFS_SETUID_FLAGS;
+	}
+
 	if (getenv("PASSWD")) {
 		if(mountpassword == NULL)
 			mountpassword = (char *)calloc(MOUNT_PASSWD_SIZE+1,1);
@@ -1283,6 +1415,27 @@ int main(int argc, char ** argv)
                 rc = EX_USAGE;
 		goto mount_exit;
 	}
+
+	if (getuid()) {
+#if !CIFS_LEGACY_SETUID_CHECK
+		if (!(flags & (MS_USERS|MS_USER))) {
+			fprintf(stderr, "%s: permission denied\n", thisprogram);
+			rc = EX_USAGE;
+			goto mount_exit;
+		}
+#endif /* !CIFS_LEGACY_SETUID_CHECK */
+
+		if (geteuid()) {
+			fprintf(stderr, "%s: not installed setuid - \"user\" "
+					"CIFS mounts not supported.",
+					thisprogram);
+			rc = EX_FAIL;
+			goto mount_exit;
+		}
+	}
+
+	flags &= ~(MS_USERS|MS_USER);
+
 	addrhead = addr = parse_server(&share_name);
 	if((addrhead == NULL) && (got_ip == 0)) {
 		printf("No ip address specified and hostname not found\n");
@@ -1299,37 +1452,6 @@ int main(int argc, char ** argv)
 			mountpoint = resolved_path; 
 		}
 	}
-	if(chdir(mountpoint)) {
-		printf("mount error: can not change directory into mount target %s\n",mountpoint);
-		rc = EX_USAGE;
-		goto mount_exit;
-	}
-
-	if(stat (".", &statbuf)) {
-		printf("mount error: mount point %s does not exist\n",mountpoint);
-		rc = EX_USAGE;
-		goto mount_exit;
-	}
-
-	if (S_ISDIR(statbuf.st_mode) == 0) {
-		printf("mount error: mount point %s is not a directory\n",mountpoint);
-		rc = EX_USAGE;
-		goto mount_exit;
-	}
-
-	if((getuid() != 0) && (geteuid() == 0)) {
-		if((statbuf.st_uid == getuid()) && (S_IRWXU == (statbuf.st_mode & S_IRWXU))) {
-#ifndef CIFS_ALLOW_USR_SUID
-			/* Do not allow user mounts to control suid flag
-			for mount unless explicitly built that way */
-			flags |= MS_NOSUID | MS_NODEV;
-#endif						
-		} else {
-			printf("mount error: permission denied or not superuser and mount.cifs not installed SUID\n"); 
-			exit(EX_USAGE);
-		}
-	}
-
 	if(got_user == 0) {
 		/* Note that the password will not be retrieved from the
 		   USER env variable (ie user%password form) as there is