[patch 09/10] unprivileged mounts: propagation: inherit owner from parent

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi@xxxxxxx>

On mount propagation, let the owner of the clone be inherited from the
parent into which it has been propagated.

If the parent has the "nosuid" flag, set this flag for the child as
well.  This is needed for the suid-less namespace (use case #2 in the
first patch header), where all mounts are owned by the user and have
the nosuid flag set.  In this case the propagated mount needs to have
nosuid, otherwise a suid executable may be misused by the user.

Similar treatment is not needed for "nodev", because devices can't be
abused this way: the user is not able to gain privileges to devices by
rearranging the mount namespace.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2008-01-16 13:25:09.000000000 +0100
+++ linux/fs/namespace.c	2008-01-16 13:25:11.000000000 +0100
@@ -506,10 +506,10 @@ static int reserve_user_mount(void)
 	return err;
 }
 
-static void __set_mnt_user(struct vfsmount *mnt)
+static void __set_mnt_user(struct vfsmount *mnt, uid_t owner)
 {
 	WARN_ON(mnt->mnt_flags & MNT_USER);
-	mnt->mnt_uid = current->fsuid;
+	mnt->mnt_uid = owner;
 	mnt->mnt_flags |= MNT_USER;
 
 	if (!capable(CAP_SETUID))
@@ -520,7 +520,7 @@ static void __set_mnt_user(struct vfsmou
 
 static void set_mnt_user(struct vfsmount *mnt)
 {
-	__set_mnt_user(mnt);
+	__set_mnt_user(mnt, current->fsuid);
 	spin_lock(&vfsmount_lock);
 	nr_user_mounts++;
 	spin_unlock(&vfsmount_lock);
@@ -536,7 +536,7 @@ static void clear_mnt_user(struct vfsmou
 }
 
 static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
-					int flag)
+					int flag, uid_t owner)
 {
 	struct super_block *sb = old->mnt_sb;
 	struct vfsmount *mnt;
@@ -560,7 +560,10 @@ 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, owner);
+
+	if (flag & CL_NOSUID)
+		mnt->mnt_flags |= MNT_NOSUID;
 
 	if (flag & CL_SLAVE) {
 		list_add(&mnt->mnt_slave, &old->mnt_slave_list);
@@ -1066,7 +1069,7 @@ static int lives_below_in_same_fs(struct
 }
 
 struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
-					int flag)
+					int flag, uid_t owner)
 {
 	struct vfsmount *res, *p, *q, *r, *s;
 	struct nameidata nd;
@@ -1074,7 +1077,7 @@ struct vfsmount *copy_tree(struct vfsmou
 	if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(mnt))
 		return ERR_PTR(-EPERM);
 
-	res = q = clone_mnt(mnt, dentry, flag);
+	res = q = clone_mnt(mnt, dentry, flag, owner);
 	if (IS_ERR(q))
 		goto error;
 	q->mnt_mountpoint = mnt->mnt_mountpoint;
@@ -1096,7 +1099,7 @@ struct vfsmount *copy_tree(struct vfsmou
 			p = s;
 			nd.path.mnt = q;
 			nd.path.dentry = p->mnt_mountpoint;
-			q = clone_mnt(p, p->mnt_root, flag);
+			q = clone_mnt(p, p->mnt_root, flag, owner);
 			if (IS_ERR(q))
 				goto error;
 			spin_lock(&vfsmount_lock);
@@ -1121,7 +1124,7 @@ struct vfsmount *collect_mounts(struct v
 {
 	struct vfsmount *tree;
 	down_read(&namespace_sem);
-	tree = copy_tree(mnt, dentry, CL_COPY_ALL | CL_PRIVATE);
+	tree = copy_tree(mnt, dentry, CL_COPY_ALL | CL_PRIVATE, 0);
 	up_read(&namespace_sem);
 	return tree;
 }
@@ -1292,7 +1295,8 @@ static int do_change_type(struct nameida
  */
 static int do_loopback(struct nameidata *nd, char *old_name, int flags)
 {
-	int clone_fl;
+	int clone_fl = 0;
+	uid_t owner = 0;
 	struct nameidata old_nd;
 	struct vfsmount *mnt = NULL;
 	int err;
@@ -1313,11 +1317,17 @@ static int do_loopback(struct nameidata 
 	if (!check_mnt(nd->path.mnt) || !check_mnt(old_nd.path.mnt))
 		goto out;
 
-	clone_fl = (flags & MS_SETUSER) ? CL_SETUSER : 0;
+	if (flags & MS_SETUSER) {
+		clone_fl |= CL_SETUSER;
+		owner = current->fsuid;
+	}
+
 	if (flags & MS_REC)
-		mnt = copy_tree(old_nd.path.mnt, old_nd.path.dentry, clone_fl);
+		mnt = copy_tree(old_nd.path.mnt, old_nd.path.dentry, clone_fl,
+				owner);
 	else
-		mnt = clone_mnt(old_nd.path.mnt, old_nd.path.dentry, clone_fl);
+		mnt = clone_mnt(old_nd.path.mnt, old_nd.path.dentry, clone_fl,
+				owner);
 
 	err = PTR_ERR(mnt);
 	if (IS_ERR(mnt))
@@ -1541,7 +1551,7 @@ static int do_new_mount(struct nameidata
 	}
 
 	if (flags & MS_SETUSER)
-		__set_mnt_user(mnt);
+		__set_mnt_user(mnt, current->fsuid);
 
 	return do_add_mount(mnt, nd, mnt_flags, NULL);
 
@@ -1937,7 +1947,7 @@ static struct mnt_namespace *dup_mnt_ns(
 	down_write(&namespace_sem);
 	/* First pass: copy the tree topology */
 	new_ns->root = copy_tree(mnt_ns->root, mnt_ns->root->mnt_root,
-					CL_COPY_ALL | CL_EXPIRE);
+					CL_COPY_ALL | CL_EXPIRE, 0);
 	if (IS_ERR(new_ns->root)) {
 		up_write(&namespace_sem);
 		kfree(new_ns);
Index: linux/fs/pnode.c
===================================================================
--- linux.orig/fs/pnode.c	2008-01-16 13:25:07.000000000 +0100
+++ linux/fs/pnode.c	2008-01-16 13:25:11.000000000 +0100
@@ -181,15 +181,28 @@ int propagate_mnt(struct vfsmount *dest_
 
 	for (m = propagation_next(dest_mnt, dest_mnt); m;
 			m = propagation_next(m, dest_mnt)) {
-		int type;
+		int clflags;
+		uid_t owner = 0;
 		struct vfsmount *source;
 
 		if (IS_MNT_NEW(m))
 			continue;
 
-		source =  get_source(m, prev_dest_mnt, prev_src_mnt, &type);
+		source =  get_source(m, prev_dest_mnt, prev_src_mnt, &clflags);
 
-		child = copy_tree(source, source->mnt_root, type);
+		if (m->mnt_flags & MNT_USER) {
+			clflags |= CL_SETUSER;
+			owner = m->mnt_uid;
+
+			/*
+			 * If propagating into a user mount which doesn't
+			 * allow suid, then make sure, the child(ren) won't
+			 * allow suid either
+			 */
+			if (m->mnt_flags & MNT_NOSUID)
+				clflags |= CL_NOSUID;
+		}
+		child = copy_tree(source, source->mnt_root, clflags, owner);
 		if (IS_ERR(child)) {
 			ret = PTR_ERR(child);
 			list_splice(tree_list, tmp_list.prev);
Index: linux/fs/pnode.h
===================================================================
--- linux.orig/fs/pnode.h	2008-01-16 13:25:05.000000000 +0100
+++ linux/fs/pnode.h	2008-01-16 13:25:11.000000000 +0100
@@ -24,6 +24,7 @@
 #define CL_PROPAGATION 		0x10
 #define CL_PRIVATE 		0x20
 #define CL_SETUSER		0x40
+#define CL_NOSUID		0x80
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {
@@ -36,4 +37,6 @@ int propagate_mnt(struct vfsmount *, str
 		struct list_head *);
 int propagate_umount(struct list_head *);
 int propagate_mount_busy(struct vfsmount *, int);
+struct vfsmount *copy_tree(struct vfsmount *, struct dentry *, int, uid_t);
+
 #endif /* _LINUX_PNODE_H */
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h	2008-01-16 13:25:09.000000000 +0100
+++ linux/include/linux/fs.h	2008-01-16 13:25:11.000000000 +0100
@@ -1493,7 +1493,6 @@ extern int may_umount(struct vfsmount *)
 extern void umount_tree(struct vfsmount *, int, struct list_head *);
 extern void release_mounts(struct list_head *);
 extern long do_mount(char *, char *, char *, unsigned long, void *);
-extern struct vfsmount *copy_tree(struct vfsmount *, struct dentry *, int);
 extern void mnt_set_mountpoint(struct vfsmount *, struct dentry *,
 				  struct vfsmount *);
 extern struct vfsmount *collect_mounts(struct vfsmount *, struct dentry *);

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