fork: Add task stack refcounting sanity check and prevent premature task stack freeing

If something goes wrong with task stack refcounting and a stack
refcount hits zero too early, warn and leak it rather than
potentially freeing it early (and silently).

Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/f29119c783a9680a4b4656e751b6123917ace94b.1477926663.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

Author: amluto

kernel/fork.c

@@ -315,6 +315,9 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 
 static void release_task_stack(struct task_struct *tsk)
 {
+	if (WARN_ON(tsk->state != TASK_DEAD))
+		return;  /* Better to leak the stack than to free prematurely */
+
 	account_kernel_stack(tsk, -1);
 	arch_release_thread_stack(tsk->stack);
 	free_thread_stack(tsk);
@@ -1862,6 +1865,7 @@ static __latent_entropy struct task_struct *copy_process(
 	atomic_dec(&p->cred->user->processes);
 	exit_creds(p);
 bad_fork_free:
+	p->state = TASK_DEAD;
 	put_task_stack(p);
 	free_task(p);
 fork_out: