From: Miklos Szeredi <mszeredi@suse.cz>

The owner doesn't need sysadmin capabilities to call umount().

Similar behavior as umount(8) on mounts having "user=UID" option in
/etc/mtab.  The difference is that umount also checks /etc/fstab,
presumably to exclude another mount on the same mountpoint.

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

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2007-04-20 11:55:05.000000000 +0200
+++ linux/fs/namespace.c	2007-04-20 11:55:06.000000000 +0200
@@ -659,6 +659,25 @@ static int do_umount(struct vfsmount *mn
 }
 
 /*
+ * umount is permitted for
+ *  - sysadmin
+ *  - mount owner, if not forced umount
+ */
+static bool permit_umount(struct vfsmount *mnt, int flags)
+{
+	if (capable(CAP_SYS_ADMIN))
+		return true;
+
+	if (!(mnt->mnt_flags & MNT_USER))
+		return false;
+
+	if (flags & MNT_FORCE)
+		return false;
+
+	return mnt->mnt_uid == current->uid;
+}
+
+/*
  * Now umount can handle mount points as well as block devices.
  * This is important for filesystems which use unnamed block devices.
  *
@@ -681,7 +700,7 @@ asmlinkage long sys_umount(char __user *
 		goto dput_and_out;
 
 	retval = -EPERM;
-	if (!capable(CAP_SYS_ADMIN))
+	if (!permit_umount(nd.mnt, flags))
 		goto dput_and_out;
 
 	retval = do_umount(nd.mnt, flags);

--
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