linux-kernel - Re: [RFCv2 09/10] x86/mm: Add userspace API to enable Linear Address Masking

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Date:   Wed, 11 May 2022 09:26:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv2 09/10] x86/mm: Add userspace API to enable Linear Address
 Masking

On Wed, May 11, 2022 at 05:27:50AM +0300, Kirill A. Shutemov wrote:
> @@ -1013,8 +1017,23 @@ static long thread_feature_prctl(struct task_struct *task, int option,
>  
>  	/* Handle ARCH_THREAD_FEATURE_ENABLE */
>  
> +	if (features & (X86_THREAD_LAM_U48 | X86_THREAD_LAM_U57)) {
> +		long ret;
> +
> +		/* LAM is only available in long mode */
> +		if (in_32bit_syscall())
> +			return -EINVAL;

So what happens if userspace sets up a 32bit code entry in the LDT and
does the LAM thing as a 64bit syscamm but then goes run 32bit code?

> +
> +		ret = enable_lam(task, features);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	task->thread.features |= features;
>  out:
> +	/* Update CR3 to get LAM active */
> +	switch_mm(task->mm, task->mm, task);
> +
>  	return task->thread.features;
>  }
>