[patch 2/9] unprivileged mounts: allow unprivileged umount

[Miklos Szeredi <miklos@szeredi.hu>]

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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	2008-01-03 20:52:38.000000000 +0100
+++ linux/fs/namespace.c	2008-01-03 21:14:16.000000000 +0100
@@ -894,6 +894,27 @@ static int do_umount(struct vfsmount *mn
 	return retval;
 }
 
+static bool is_mount_owner(struct vfsmount *mnt, uid_t uid)
+{
+	return (mnt->mnt_flags & MNT_USER) && mnt->mnt_uid == uid;
+}
+
+/*
+ * 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 (flags & MNT_FORCE)
+		return false;
+
+	return is_mount_owner(mnt, current->fsuid);
+}
+
 /*
  * Now umount can handle mount points as well as block devices.
  * This is important for filesystems which use unnamed block devices.
@@ -917,7 +938,7 @@ asmlinkage long sys_umount(char __user *
 		goto dput_and_out;
 
 	retval = -EPERM;
-	if (!capable(CAP_SYS_ADMIN))
+	if (!permit_umount(nd.path.mnt, flags))
 		goto dput_and_out;
 
 	retval = do_umount(nd.path.mnt, flags);

--