linux-kernel - [tip: locking/urgent] futex: Initialize futex_phash

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Message-ID: <175068388403.406.13726507646292269621.tip-bot2@tip-bot2>
Date: Mon, 23 Jun 2025 13:04:44 -0000
From: "tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Calvin Owens <calvin@...nvd.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: locking/urgent] futex: Initialize futex_phash_new during fork().

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     a24cc6ce1933eade12aa2b9859de0fcd2dac2c06
Gitweb:        https://git.kernel.org/tip/a24cc6ce1933eade12aa2b9859de0fcd2dac2c06
Author:        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate:    Mon, 23 Jun 2025 10:34:08 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Mon, 23 Jun 2025 14:50:37 +02:00

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().

Closes: https://lore.kernel.org/all/aFBQ8CBKmRzEqIfS@mozart.vkv.me/
Fixes: bd54df5ea7cad ("futex: Allow to resize the private local hash")
Reported-by: Calvin Owens <calvin@...nvd.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: Calvin Owens <calvin@...nvd.org>
Link: https://lkml.kernel.org/r/20250623083408.jTiJiC6_@linutronix.de
---
 include/linux/futex.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 005b040..b371936 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);
 }