fix open/umount race

nameidata_to_filp() drops nd->path or transfers it to opened
file.  In the former case it's a Bad Idea(tm) to do mnt_drop_write()
on nd->path.mnt, since we might race with umount and vfsmount in
question might be gone already.

Fix: don't drop it, then...  IOW, have nameidata_to_filp() grab nd->path
in case it transfers it to file and do path_drop() in callers.  After
they are through with accessing nd->path...
Reported-by: NMiklos Szeredi <miklos@szeredi.hu>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1574,6 +1574,7 @@ static struct file *finish_open(struct nameidata *nd,
 	 */
 	if (will_truncate)
 		mnt_drop_write(nd->path.mnt);
+	path_put(&nd->path);
 	return filp;
 
 exit:
@@ -1675,6 +1676,7 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
 		}
 		filp = nameidata_to_filp(nd);
 		mnt_drop_write(nd->path.mnt);
+		path_put(&nd->path);
 		if (!IS_ERR(filp)) {
 			error = ima_file_check(filp, acc_mode);
 			if (error) {

fs/open.c

@@ -786,11 +786,11 @@ struct file *nameidata_to_filp(struct nameidata *nd)
 	/* Pick up the filp from the open intent */
 	filp = nd->intent.open.file;
 	/* Has the filesystem initialised the file for us? */
-	if (filp->f_path.dentry == NULL)
+	if (filp->f_path.dentry == NULL) {
+		path_get(&nd->path);
 		filp = __dentry_open(nd->path.dentry, nd->path.mnt, filp,
 				     NULL, cred);
-	else
-		path_put(&nd->path);
+	}
 	return filp;
 }