linux-kernel - [PATCH] futex: Initialize futex_phash

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Message-ID: <20250623083408.jTiJiC6_@linutronix.de>
Date: Mon, 23 Jun 2025 10:34:08 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>,
	Calvin Owens <calvin@...nvd.org>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] futex: Initialize futex_phash_new during fork().

During a hash resize operation the new private hash is stored in
mm_struct::futex_phash_new if the current hash can not be immediately
replaced.

The new hash must not be copied during fork() into the new task. Doing
so will lead to a double-free of the memory by the two tasks.

Initialize the mm_struct::futex_phash_new during fork().

Reported-by: Calvin Owens <calvin@...nvd.org>
Closes: https://lore.kernel.org/all/aFBQ8CBKmRzEqIfS@mozart.vkv.me/
Tested-by: Calvin Owens <calvin@...nvd.org>
Fixes: bd54df5ea7cad ("futex: Allow to resize the private local hash")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/futex.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 005b040c4791b..b37193653e6b5 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -89,6 +89,7 @@ void futex_hash_free(struct mm_struct *mm);
 static inline void futex_mm_init(struct mm_struct *mm)
 {
 	RCU_INIT_POINTER(mm->futex_phash, NULL);
+	mm->futex_phash_new = NULL;
 	mutex_init(&mm->futex_hash_lock);
 }
 
-- 
2.50.0