ptrace: fix exit_ptrace() vs ptrace_traceme() race

Pointed out by Roland.  The bug was recently introduced by me in
"forget_original_parent: split out the un-ptrace part", commit
39c626ae47.

Since that patch we have a window after exit_ptrace() drops tasklist and
before forget_original_parent() takes it again.  In this window the child
can do ptrace(PTRACE_TRACEME) and nobody can untrace this child after
that.

Change ptrace_traceme() to not attach to the exiting ->real_parent.  We
don't report the error in this case, we pretend we attach right before
->real_parent calls exit_ptrace() which should untrace us anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/ptrace.c

@@ -604,10 +604,11 @@ repeat:
 		ret = security_ptrace_traceme(current->parent);
 
 		/*
-		 * Set the ptrace bit in the process ptrace flags.
-		 * Then link us on our parent's ptraced list.
+		 * Check PF_EXITING to ensure ->real_parent has not passed
+		 * exit_ptrace(). Otherwise we don't report the error but
+		 * pretend ->real_parent untraces us right after return.
 		 */
-		if (!ret) {
+		if (!ret && !(current->real_parent->flags & PF_EXITING)) {
 			current->ptrace |= PT_PTRACED;
 			__ptrace_link(current, current->real_parent);
 		}