Yama: access task_struct->comm directly

The core ptrace access checking routine holds a task lock, and when
reporting a failure, Yama takes a separate task lock. To avoid a
potential deadlock with two ptracers taking the opposite locks, do not
use get_task_comm() and just use ->comm directly since accuracy is not
important for the report.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
CC: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

security/yama/yama_lsm.c

@@ -279,12 +279,9 @@ static int yama_ptrace_access_check(struct task_struct *child,
 	}
 
 	if (rc) {
-		char name[sizeof(current->comm)];
 		printk_ratelimited(KERN_NOTICE
 			"ptrace of pid %d was attempted by: %s (pid %d)\n",
-			child->pid,
-			get_task_comm(name, current),
-			current->pid);
+			child->pid, current->comm, current->pid);
 	}
 
 	return rc;
@@ -319,12 +316,9 @@ static int yama_ptrace_traceme(struct task_struct *parent)
 	}
 
 	if (rc) {
-		char name[sizeof(current->comm)];
 		printk_ratelimited(KERN_NOTICE
 			"ptraceme of pid %d was attempted by: %s (pid %d)\n",
-			current->pid,
-			get_task_comm(name, parent),
-			parent->pid);
+			current->pid, parent->comm, parent->pid);
 	}
 
 	return rc;