Re: [PATCH v3 1/2] futex: Canonicalize arm64 tagged user addresses

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Nov 09, 2026 at 10:26:47AM -0800, Shubhang Kaushik (Ampere) wrote:
> On arm64, a thread that enables the tagged address ABI can pass valid
> top-byte-tagged user pointers to futex syscalls. Such a pointer must
> have the same syscall behaviour as its untagged form.
> 
> get_futex_key() validates the user address with access_ok(), but uses
> the raw address when constructing private and shared-anonymous futex
> keys. A waiter using a tagged address and a waker using the untagged
> form can therefore access the same futex word while hashing to distinct
> keys, causing FUTEX_WAKE to miss the waiter.
> 
> Keep access_ok() on the original pointer so that arm64 retains its
> per-thread tagged-address ABI enforcement. Canonicalize the address
> afterwards, before it is used for futex key construction and shared
> mapping lookup.
> 
> Fixes: 63f0c6037965 ("arm64: Introduce prctl() options to control the tagged user addresses ABI")
> Cc: stable@vger.kernel.org
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Acked-by: Davidlohr Bueso <dave@stgolabs.net>
> Signed-off-by: Shubhang Kaushik (Ampere) <sh@gentwo.org>
> ---
>  kernel/futex/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index a240638f69e5c65bd036434a15f3df680efb4638..dcc8ffca38595a5ea69f3672375ffb9f9c6766f9 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -539,6 +539,8 @@ int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
>  	if (unlikely(!access_ok(uaddr, size)))
>  		return -EFAULT;
>  
> +	address = untagged_addr(address);
> +
>  	if (unlikely(should_fail_futex(fshared)))
>  		return -EFAULT;

Thomas, Peter, do you plan to take this fix via tip or you'd rather we
take it via arm64? TBH, I don't think it's urgent, the bug has been
around since 2019.

Thanks.

-- 
Catalin