From: Miklos Szeredi <[email-masked]>

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 <[email-masked]>
Acked-by: Serge Hallyn <[email-masked]>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2008-01-16 13:25:07.000000000 +0100
+++ linux/fs/namespace.c	2008-01-16 13:25:08.000000000 +0100
@@ -511,6 +511,11 @@ static void __set_mnt_user(struct vfsmou
 	WARN_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)
@@ -1021,22 +1026,26 @@ 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)
@@ -1280,9 +1289,10 @@ 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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