/proc/*/environ: wrong placing of ptrace_may_attach() check

It's a bit dopey-looking and can permit a task to cause a pagefault in an mm
which it doesn't have permission to read from.
Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -204,12 +204,17 @@ static int proc_pid_environ(struct task_struct *task, char * buffer)
 	int res = 0;
 	struct mm_struct *mm = get_task_mm(task);
 	if (mm) {
-		unsigned int len = mm->env_end - mm->env_start;
+		unsigned int len;
+
+		res = -ESRCH;
+		if (!ptrace_may_attach(task))
+			goto out;
+
+		len  = mm->env_end - mm->env_start;
 		if (len > PAGE_SIZE)
 			len = PAGE_SIZE;
 		res = access_process_vm(task, mm->env_start, buffer, len, 0);
-		if (!ptrace_may_attach(task))
-			res = -ESRCH;
+out:
 		mmput(mm);
 	}
 	return res;