use uniform permission checks for all mount propagation changes

do_change_type() and do_set_group() are operating on different
aspects of the same thing - propagation graph.  The latter
asks for mounts involved to be mounted in namespace(s) the caller
has CAP_SYS_ADMIN for.  The former is a mess - originally it
didn't even check that mount *is* mounted.  That got fixed,
but the resulting check turns out to be too strict for userland -
in effect, we check that mount is in our namespace, having already
checked that we have CAP_SYS_ADMIN there.

What we really need (in both cases) is
	* only touch mounts that are mounted.  That's a must-have
constraint - data corruption happens if it get violated.
	* don't allow to mess with a namespace unless you already
have enough permissions to do so (i.e. CAP_SYS_ADMIN in its userns).

That's an equivalent of what do_set_group() does; let's extract that
into a helper (may_change_propagation()) and use it in both
do_set_group() and do_change_type().

Fixes: 12f147d "do_change_type(): refuse to operate on unmounted/not ours mounts"
Acked-by: Andrei Vagin <avagin@gmail.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Tested-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/namespace.c

@@ -2859,6 +2859,19 @@ static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp)
 	return attach_recursive_mnt(mnt, p, mp);
 }
 
+static int may_change_propagation(const struct mount *m)
+{
+        struct mnt_namespace *ns = m->mnt_ns;
+
+	 // it must be mounted in some namespace
+	 if (IS_ERR_OR_NULL(ns))         // is_mounted()
+		 return -EINVAL;
+	 // and the caller must be admin in userns of that namespace
+	 if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
+		 return -EPERM;
+	 return 0;
+}
+
 /*
  * Sanity check the flags to change_mnt_propagation.
  */
@@ -2895,10 +2908,10 @@ static int do_change_type(struct path *path, int ms_flags)
 		return -EINVAL;
 
 	namespace_lock();
-	if (!check_mnt(mnt)) {
-		err = -EINVAL;
+	err = may_change_propagation(mnt);
+	if (err)
 		goto out_unlock;
-	}
+
 	if (type == MS_SHARED) {
 		err = invent_group_ids(mnt, recurse);
 		if (err)
@@ -3344,18 +3357,11 @@ static int do_set_group(struct path *from_path, struct path *to_path)
 
 	namespace_lock();
 
-	err = -EINVAL;
-	/* To and From must be mounted */
-	if (!is_mounted(&from->mnt))
-		goto out;
-	if (!is_mounted(&to->mnt))
-		goto out;
-
-	err = -EPERM;
-	/* We should be allowed to modify mount namespaces of both mounts */
-	if (!ns_capable(from->mnt_ns->user_ns, CAP_SYS_ADMIN))
+	err = may_change_propagation(from);
+	if (err)
 		goto out;
-	if (!ns_capable(to->mnt_ns->user_ns, CAP_SYS_ADMIN))
+	err = may_change_propagation(to);
+	if (err)
 		goto out;
 
 	err = -EINVAL;