[tip:core/urgent] fork: Add task stack refcounting sanity check and prevent premature task stack freeing

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  405c0759712f57b680f66aee9c55cd06ad1cbdef
Gitweb:     http://git.kernel.org/tip/405c0759712f57b680f66aee9c55cd06ad1cbdef
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Mon, 31 Oct 2016 08:11:43 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 1 Nov 2016 07:39:17 +0100

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 <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/f29119c783a9680a4b4656e751b6123917ace94b.1477926663.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/fork.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index 623259f..997ac1d 100644
--- a/kernel/fork.c
+++ b/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 @@ bad_fork_cleanup_count:
 	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: