close race in /proc/*/environ

Switch to mm_for_maps().  Maybe we ought to make it r--r--r--,
since we do checks on IO anyway...
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/proc/base.c

@@ -919,20 +919,18 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 	if (!task)
 		goto out_no_task;
 
-	if (!ptrace_may_access(task, PTRACE_MODE_READ))
-		goto out;
-
 	ret = -ENOMEM;
 	page = (char *)__get_free_page(GFP_TEMPORARY);
 	if (!page)
 		goto out;
 
-	ret = 0;
 
-	mm = get_task_mm(task);
-	if (!mm)
+	mm = mm_for_maps(task);
+	ret = PTR_ERR(mm);
+	if (!mm || IS_ERR(mm))
 		goto out_free;
 
+	ret = 0;
 	while (count > 0) {
 		int this_len, retval, max_len;