Patchwork [17/19] mount.cifs: guard against signals by unprivileged users

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Submitter Jeff Layton
Date March 26, 2010, 2:25 p.m.
Message ID <1269613542-6402-18-git-send-email-jlayton@samba.org>
Download mbox | patch
Permalink /patch/48649/
State New
Headers show

Comments

Jeff Layton - March 26, 2010, 2:25 p.m.
From: Jeff Layton <jlayton@redhat.com>

If mount.cifs is setuid root, then the unprivileged user who runs the
program can send the mount.cifs process a signal and kill it. This is
not a huge problem unless we happen to be updating the mtab at the
time, in which case the mtab lockfiles might not get cleaned up.

To remedy this, have the privileged mount.cifs process set its real
uid to the effective uid (usually, root). This prevents unprivileged
users from being able to signal the process.

While we're at it, also mask off signals while we're updating the
mtab. This leaves a SIGKILL by root as the only way to interrupt the
mtab update, but there's really nothing we can do about that.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 mount.cifs.c |   42 +++++++++++++++++++++++++++++++++++-------
 1 files changed, 35 insertions(+), 7 deletions(-)

Patch

diff --git a/mount.cifs.c b/mount.cifs.c
index f4aea01..e292e40 100644
--- a/mount.cifs.c
+++ b/mount.cifs.c
@@ -267,10 +267,10 @@  static int set_password(struct parsed_mount_info *parsed_info, const char *src)
 }
 
 /* caller frees username if necessary */
-static char *getusername(void)
+static char *getusername(uid_t uid)
 {
 	char *username = NULL;
-	struct passwd *password = getpwuid(getuid());
+	struct passwd *password = getpwuid(uid);
 
 	if (password)
 		username = password->pw_name;
@@ -1051,13 +1051,26 @@  static int check_mtab(const char *progname, const char *devname,
 }
 
 static int
-add_mtab(char *devname, char *mountpoint, unsigned long flags)
+add_mtab(char *devname, char *mountpoint, unsigned long flags, uid_t uid)
 {
 	int rc = 0;
 	char *mount_user;
 	struct mntent mountent;
 	FILE *pmntfile;
+	sigset_t mask, oldmask;
 
+	rc = sigfillset(&mask);
+	if (rc) {
+		fprintf(stderr, "Unable to set filled signal mask\n");
+		return EX_FILEIO;
+	}
+
+	rc = sigprocmask(SIG_SETMASK, &mask, &oldmask);
+	if (rc) {
+		fprintf(stderr, "Unable to make process ignore signals\n");
+		return EX_FILEIO;
+	}
+	
 	atexit(unlock_mtab);
 	rc = lock_mtab();
 	if (rc) {
@@ -1094,9 +1107,9 @@  add_mtab(char *devname, char *mountpoint, unsigned long flags)
 			strlcat(mountent.mnt_opts, ",nodev", MTAB_OPTIONS_LEN);
 		if (flags & MS_SYNCHRONOUS)
 			strlcat(mountent.mnt_opts, ",sync", MTAB_OPTIONS_LEN);
-		if (getuid() != 0) {
+		if (uid != 0) {
 			strlcat(mountent.mnt_opts, ",user=", MTAB_OPTIONS_LEN);
-			mount_user = getusername();
+			mount_user = getusername(uid);
 			if (mount_user)
 				strlcat(mountent.mnt_opts, mount_user,
 					MTAB_OPTIONS_LEN);
@@ -1109,6 +1122,7 @@  add_mtab(char *devname, char *mountpoint, unsigned long flags)
 	unlock_mtab();
 	SAFE_FREE(mountent.mnt_opts);
 add_mtab_exit:
+	sigprocmask(SIG_SETMASK, &oldmask, NULL);
 	if (rc) {
 		fprintf(stderr, "unable to add mount entry to mtab\n");
 		rc = EX_FILEIO;
@@ -1204,7 +1218,7 @@  assemble_mountinfo(struct parsed_mount_info *parsed_info,
 			strlcpy(parsed_info->username, getenv("USER"),
 				sizeof(parsed_info->username));
 		else
-			strlcpy(parsed_info->username, getusername(),
+			strlcpy(parsed_info->username, getusername(getuid()),
 				sizeof(parsed_info->username));
 		parsed_info->got_user = 1;
 	}
@@ -1259,6 +1273,7 @@  int main(int argc, char **argv)
 	size_t dev_len;
 	struct parsed_mount_info *parsed_info = NULL;
 	pid_t pid;
+	uid_t uid;
 
 	if (check_setuid())
 		return EX_USAGE;
@@ -1390,6 +1405,19 @@  int main(int argc, char **argv)
 			goto mount_exit;
 	}
 
+	/*
+	 * Set the real uid to the effective uid. This prevents unprivileged
+	 * users from sending signals to this process, though ^c on controlling
+	 * terminal should still work.
+	 */
+	uid = getuid();
+	rc = setreuid(geteuid(), -1);
+	if (rc != 0) {
+		fprintf(stderr, "Unable to set real uid to effective uid: %s\n",
+				strerror(errno));
+		rc = EX_USAGE;
+	}
+
 	options = calloc(options_size, 1);
 	if (!options) {
 		fprintf(stderr, "Unable to allocate memory.\n");
@@ -1500,7 +1528,7 @@  mount_retry:
 	}
 
 	if (!parsed_info->nomtab)
-		rc = add_mtab(dev_name, mountpoint, parsed_info->flags);
+		rc = add_mtab(dev_name, mountpoint, parsed_info->flags, uid);
 
 mount_exit:
 	if (parsed_info) {