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

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Message-ID: <20250830222545.GAaLN6aeQzWHYc20BF@fat_crate.local>
Date: Sun, 31 Aug 2025 00:25:45 +0200
From: Borislav Petkov <bp@...en8.de>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	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: Re: [PATCH] futex: Initialize mm_struct::futex_ref on mm_init()

On Sat, Aug 30, 2025 at 11:38:06PM +0200, Sebastian Andrzej Siewior wrote:
> 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()")

So this one is queued in locking/urgent and I guess I can fold this new one into
it so that that commit is whole and fixed.

And then leave it in that branch for one more week for some more testing...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette