From: Miklos Szeredi <mszeredi@suse.cz>

Allow bind mounts to unprivileged users if the following conditions are met:

  - mountpoint is not a symlink
  - parent mount is owned by the user
  - the number of user mounts is below the maximum

Unprivileged mounts imply MS_SETUSER, and will also have the "nosuid" and
"nodev" mount flags set.

In particular, if mounting process doesn't have CAP_SETUID capability,
then the "nosuid" flag will be added, and if it doesn't have CAP_MKNOD
capability, then the "nodev" flag will be added.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2008-01-04 13:47:49.000000000 +0100
+++ linux/fs/namespace.c	2008-01-04 13:48:01.000000000 +0100
@@ -487,11 +487,34 @@ static void dec_nr_user_mounts(void)
 	spin_unlock(&vfsmount_lock);
 }
 
-static void set_mnt_user(struct vfsmount *mnt)
+static int reserve_user_mount(void)
+{
+	int err = 0;
+
+	spin_lock(&vfsmount_lock);
+	if (nr_user_mounts >= max_user_mounts && !capable(CAP_SYS_ADMIN))
+		err = -EPERM;
+	else
+		nr_user_mounts++;
+	spin_unlock(&vfsmount_lock);
+	return err;
+}
+
+static void __set_mnt_user(struct vfsmount *mnt)
 {
 	BUG_ON(mnt->mnt_flags & MNT_USER);
 	mnt->mnt_uid = current->fsuid;
 	mnt->mnt_flags |= MNT_USER;
+
+	if (!capable(CAP_SETUID))
+		mnt->mnt_flags |= MNT_NOSUID;
+	if (!capable(CAP_MKNOD))
+		mnt->mnt_flags |= MNT_NODEV;
+}
+
+static void set_mnt_user(struct vfsmount *mnt)
+{
+	__set_mnt_user(mnt);
 	spin_lock(&vfsmount_lock);
 	nr_user_mounts++;
 	spin_unlock(&vfsmount_lock);
@@ -510,10 +533,16 @@ static struct vfsmount *clone_mnt(struct
 					int flag)
 {
 	struct super_block *sb = old->mnt_sb;
-	struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname);
+	struct vfsmount *mnt;
 
+	if (flag & CL_SETUSER) {
+		int err = reserve_user_mount();
+		if (err)
+			return ERR_PTR(err);
+	}
+	mnt = alloc_vfsmnt(old->mnt_devname);
 	if (!mnt)
-		return ERR_PTR(-ENOMEM);
+		goto alloc_failed;
 
 	mnt->mnt_flags = old->mnt_flags;
 	atomic_inc(&sb->s_active);
@@ -525,7 +554,7 @@ static struct vfsmount *clone_mnt(struct
 	/* don't copy the MNT_USER flag */
 	mnt->mnt_flags &= ~MNT_USER;
 	if (flag & CL_SETUSER)
-		set_mnt_user(mnt);
+		__set_mnt_user(mnt);
 
 	if (flag & CL_SLAVE) {
 		list_add(&mnt->mnt_slave, &old->mnt_slave_list);
@@ -550,6 +579,11 @@ static struct vfsmount *clone_mnt(struct
 		spin_unlock(&vfsmount_lock);
 	}
 	return mnt;
+
+ alloc_failed:
+	if (flag & CL_SETUSER)
+		dec_nr_user_mounts();
+	return ERR_PTR(-ENOMEM);
 }
 
 static inline void __mntput(struct vfsmount *mnt)
@@ -986,22 +1020,26 @@ asmlinkage long sys_oldumount(char __use
 
 #endif
 
-static int mount_is_safe(struct nameidata *nd)
+/*
+ * Conditions for unprivileged mounts are:
+ * - mountpoint is not a symlink
+ * - mountpoint is in a mount owned by the user
+ */
+static bool permit_mount(struct nameidata *nd, int *flags)
 {
+	struct inode *inode = nd->path.dentry->d_inode;
+
 	if (capable(CAP_SYS_ADMIN))
-		return 0;
-	return -EPERM;
-#ifdef notyet
-	if (S_ISLNK(nd->path.dentry->d_inode->i_mode))
-		return -EPERM;
-	if (nd->path.dentry->d_inode->i_mode & S_ISVTX) {
-		if (current->uid != nd->path.dentry->d_inode->i_uid)
-			return -EPERM;
-	}
-	if (vfs_permission(nd, MAY_WRITE))
-		return -EPERM;
-	return 0;
-#endif
+		return true;
+
+	if (S_ISLNK(inode->i_mode))
+		return false;
+
+	if (!is_mount_owner(nd->path.mnt, current->fsuid))
+		return false;
+
+	*flags |= MS_SETUSER;
+	return true;
 }
 
 static int lives_below_in_same_fs(struct dentry *d, struct dentry *dentry)
@@ -1245,9 +1283,10 @@ static int do_loopback(struct nameidata 
 	int clone_fl;
 	struct nameidata old_nd;
 	struct vfsmount *mnt = NULL;
-	int err = mount_is_safe(nd);
-	if (err)
-		return err;
+	int err;
+
+	if (!permit_mount(nd, &flags))
+		return -EPERM;
 	if (!old_name || !*old_name)
 		return -EINVAL;
 	err = path_lookup(old_name, LOOKUP_FOLLOW, &old_nd);

--
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