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From: Thomas Gleixner <tglx@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Hao-Yu Yang <naup96721@gmail.com>, mingo@redhat.com,
 linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
 David Hillenbrand <david@kernel.org>, Eric Dumazet <edumazet@google.com>,
 linux-mm@kvack.org
Subject: Re: [PATCH v2] futex: Use-after-free between futex_key_to_node_opt
 and vma_replace_policy
In-Reply-To: <20260324140019.GE3738010@noisy.programming.kicks-ass.net>
References: <20260313124756.52461-1-naup96721@gmail.com>
 <87a4vyihlx.ffs@tglx>
 <20260324140019.GE3738010@noisy.programming.kicks-ass.net>
Date: Tue, 24 Mar 2026 17:36:42 +0100
Message-ID: <87fr5pgp5x.ffs@tglx>
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On Tue, Mar 24 2026 at 15:00, Peter Zijlstra wrote:
> On Mon, Mar 23, 2026 at 06:24:42PM +0100, Thomas Gleixner wrote:
> Not to mention we don't actually need any of that here, because:
>
>> Especially the writer side is required so that the proper memory
>> barriers are inserted for architectures with a weakly ordered memory
>> model.
>
> The vma->vm_policy thing is written under mmap_lock held for writing,
> and the futex consumer is a speculative read lock. Specifically the
> ordering is through the associated seqcount.

Duh. Yes.

> All that is really needed is to extend the lifetime of the mpol to the
> associated RCU period. Which is exactly what this patch does.
>
> Want me to go write up a better Changelog?

And a comment in the code explaining the RCU magic perhaps?