linux-kernel - [PATCH] fork: Prevent premature task stack freeing

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Message-Id: <f29119c783a9680a4b4656e751b6123917ace94b.1477926663.git.luto@kernel.org>
Date:   Mon, 31 Oct 2016 08:11:43 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] fork: 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.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 kernel/fork.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index 623259fc794d..997ac1d584f7 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 @@ 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:
-- 
2.7.4