linux-kernel - [PATCH] futex: Initialize mm_struct::futex_ref on mm

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Message-ID: <20250830213806.sEKuuGSm@linutronix.de>
Date: Sat, 30 Aug 2025 23:38:06 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>,
	Borislav Petkov <bp@...en8.de>, 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 mm_struct::futex_ref on mm_init()

mm_struct::futex_ref needs to be set to NULL during mm_init() to ensure
that it is not copied during fork(). Reusing the previous pointer can
lead to use-after-free which can trigger

| WARNING: kernel/futex/core.c:1604 at futex_ref_rcu+0xe1/0xf0, CPU#2: swapper/2/0

Initialize mm_struct::futex_ref on mm_init().

Fixes: 1b708b38414d3 ("futex: Move futex_hash_free() back to __mmput()")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/futex/core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index fb63c13aa66fc..125804fbb5cb1 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1722,6 +1722,7 @@ int futex_mm_init(struct mm_struct *mm)
 	RCU_INIT_POINTER(mm->futex_phash, NULL);
 	mm->futex_phash_new = NULL;
 	/* futex-ref */
+	mm->futex_ref = NULL;
 	atomic_long_set(&mm->futex_atomic, 0);
 	mm->futex_batches = get_state_synchronize_rcu();
 	return 0;
-- 
2.51.0