[patch 7/8] allow unprivileged mounts

[Miklos Szeredi <miklos@xxxxxxxxxx>]

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.

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

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2007-04-04 19:30:02.000000000 +0200
+++ linux/fs/namespace.c	2007-04-04 19:30:08.000000000 +0200
@@ -757,11 +757,15 @@ asmlinkage long sys_oldumount(char __use
 
 #endif
 
-static int mount_is_safe(struct nameidata *nd, int *flags)
+static int mount_is_safe(struct nameidata *nd, struct file_system_type *type,
+			 int *flags)
 {
 	if (capable(CAP_SYS_ADMIN))
 		return 0;
 
+	if (type && !(type->fs_flags & FS_SAFE))
+		return -EPERM;
+
 	if (!S_ISDIR(nd->dentry->d_inode->i_mode) &&
 	    !S_ISREG(nd->dentry->d_inode->i_mode))
 		return -EPERM;
@@ -994,7 +998,7 @@ static int do_loopback(struct nameidata 
 	int clone_flags;
 	struct nameidata old_nd;
 	struct vfsmount *mnt = NULL;
-	int err = mount_is_safe(nd, &flags);
+	int err = mount_is_safe(nd, NULL, &flags);
 	if (err)
 		return err;
 	if (!old_name || !*old_name)
@@ -1156,26 +1160,46 @@ 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;
+	type = get_fs_type(fstype);
+	if (!type)
+		return -ENODEV;
 
-	mnt = do_kern_mount(type, flags & ~MS_SETUSER, name, data);
-	if (IS_ERR(mnt))
+	err = mount_is_safe(nd, type, &flags);
+	if (err)
+		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;
 }
 
 /*
@@ -1205,7 +1229,7 @@ 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	2007-04-04 19:30:05.000000000 +0200
+++ linux/include/linux/fs.h	2007-04-04 19:30:08.000000000 +0200
@@ -95,6 +95,7 @@ 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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