Re: [PATCH v2] futex: Use-after-free between futex_key_to_node_opt and vma_replace_policy

[Thomas Gleixner]

On Tue, Mar 24 2026 at 18:44, Peter Zijlstra wrote:

> On Tue, Mar 24, 2026 at 05:36:42PM +0100, Thomas Gleixner wrote:
>> 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?
>
> Does this work for you?

Perfect

> ---
> Subject: futex: Fix UaF between futex_key_to_node_opt() and vma_replace_policy()
> From: Hao-Yu Yang <naup96721@xxxxxxxxx>
> Date: Fri, 13 Mar 2026 20:47:56 +0800
>
> From: Hao-Yu Yang <naup96721@xxxxxxxxx>
>
> During futex_key_to_node_opt() execution, vma->vm_policy is read under
> speculative mmap lock and RCU. Concurrently, mbind() may call
> vma_replace_policy() which frees the old mempolicy immediately via
> kmem_cache_free().
>
> This creates a race where __futex_key_to_node() dereferences a freed
> mempolicy pointer, causing a use-after-free read of mpol->mode.
>
> [  151.412631] BUG: KASAN: slab-use-after-free in __futex_key_to_node (kernel/futex/core.c:349)
> [  151.414046] Read of size 2 at addr ffff888001c49634 by task e/87
>
> [  151.415969] Call Trace:
>
> [  151.416732]  __asan_load2 (mm/kasan/generic.c:271)
> [  151.416777]  __futex_key_to_node (kernel/futex/core.c:349)
> [  151.416822]  get_futex_key (kernel/futex/core.c:374 kernel/futex/core.c:386 kernel/futex/core.c:593)
>
> Fix by adding rcu to __mpol_put().
>
> Fixes: c042c505210d ("futex: Implement FUTEX2_MPOL")
> Reported-by: Hao-Yu Yang <naup96721@xxxxxxxxx>
> Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Signed-off-by: Hao-Yu Yang <naup96721@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxx>

I think there was also a Reviewed-by from Eric in one of the previous threads.

Thanks,

        tglx