[patch 07/10] unprivileged mounts: allow unprivileged mounts

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi@xxxxxxx>

Define a new fs flag FS_SAFE, which denotes, that unprivileged mounting of
this filesystem may not constitute a security problem.

Since most filesystems haven't been designed with unprivileged mounting in
mind, a thorough audit is needed before setting this flag.

For "safe" filesystems also allow unprivileged forced unmounting.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2007-04-26 13:30:04.000000000 +0200
+++ linux/fs/namespace.c	2007-04-26 13:51:29.000000000 +0200
@@ -724,14 +724,16 @@ static bool is_mount_owner(struct vfsmou
 /*
  * umount is permitted for
  *  - sysadmin
- *  - mount owner, if not forced umount
+ *  - mount owner
+ *    o if not forced umount,
+ *    o if forced umount, and filesystem is "safe"
  */
 static bool permit_umount(struct vfsmount *mnt, int flags)
 {
 	if (capable(CAP_SYS_ADMIN))
 		return true;
 
-	if (flags & MNT_FORCE)
+	if ((flags & MNT_FORCE) && !(mnt->mnt_sb->s_type->fs_flags & FS_SAFE))
 		return false;
 
 	return is_mount_owner(mnt, current->fsuid);
@@ -787,13 +789,17 @@ asmlinkage long sys_oldumount(char __use
  * - mountpoint is not a symlink
  * - mountpoint is in a mount owned by the user
  */
-static bool permit_mount(struct nameidata *nd, int *flags)
+static bool permit_mount(struct nameidata *nd, struct file_system_type *type,
+			 int *flags)
 {
 	struct inode *inode = nd->dentry->d_inode;
 
 	if (capable(CAP_SYS_ADMIN))
 		return true;
 
+	if (type && !(type->fs_flags & FS_SAFE))
+		return false;
+
 	if (S_ISLNK(inode->i_mode))
 		return false;
 
@@ -1027,7 +1033,7 @@ static int do_loopback(struct nameidata 
 	struct vfsmount *mnt = NULL;
 	int err;
 
-	if (!permit_mount(nd, &flags))
+	if (!permit_mount(nd, NULL, &flags))
 		return -EPERM;
 	if (!old_name || !*old_name)
 		return -EINVAL;
@@ -1188,26 +1194,46 @@ out:
  * create a new mount for userspace and request it to be added into the
  * namespace's tree
  */
-static int do_new_mount(struct nameidata *nd, char *type, int flags,
+static int do_new_mount(struct nameidata *nd, char *fstype, int flags,
 			int mnt_flags, char *name, void *data)
 {
+	int err;
 	struct vfsmount *mnt;
+	struct file_system_type *type;
 
-	if (!type || !memchr(type, 0, PAGE_SIZE))
+	if (!fstype || !memchr(fstype, 0, PAGE_SIZE))
 		return -EINVAL;
 
-	/* we need capabilities... */
-	if (!capable(CAP_SYS_ADMIN))
-		return -EPERM;
-
-	mnt = do_kern_mount(type, flags & ~MS_SETUSER, name, data);
-	if (IS_ERR(mnt))
+	type = get_fs_type(fstype);
+	if (!type)
+		return -ENODEV;
+
+	err = -EPERM;
+	if (!permit_mount(nd, type, &flags))
+		goto out_put_filesystem;
+
+	if (flags & MS_SETUSER) {
+		err = reserve_user_mount();
+		if (err)
+			goto out_put_filesystem;
+	}
+
+	mnt = vfs_kern_mount(type, flags & ~MS_SETUSER, name, data);
+	put_filesystem(type);
+	if (IS_ERR(mnt)) {
+		if (flags & MS_SETUSER)
+			dec_nr_user_mounts();
 		return PTR_ERR(mnt);
+	}
 
 	if (flags & MS_SETUSER)
-		set_mnt_user(mnt);
+		__set_mnt_user(mnt);
 
 	return do_add_mount(mnt, nd, mnt_flags, NULL);
+
+ out_put_filesystem:
+	put_filesystem(type);
+	return err;
 }
 
 /*
@@ -1237,7 +1263,7 @@ int do_add_mount(struct vfsmount *newmnt
 	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
 		goto unlock;
 
-	/* MNT_USER was set earlier */
+	/* some flags may have been set earlier */
 	newmnt->mnt_flags |= mnt_flags;
 	if ((err = graft_tree(newmnt, nd)))
 		goto unlock;
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h	2007-04-26 13:46:26.000000000 +0200
+++ linux/include/linux/fs.h	2007-04-26 13:48:14.000000000 +0200
@@ -96,6 +96,7 @@ extern int dir_notify_enable;
 #define FS_REQUIRES_DEV 1 
 #define FS_BINARY_MOUNTDATA 2
 #define FS_HAS_SUBTYPE 4
+#define FS_SAFE 8		/* Safe to mount by unprivileged users */
 #define FS_REVAL_DOT	16384	/* Check the paths ".", ".." for staleness */
 #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move()
 					 * during rename() internally.

--
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