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Date:   Wed, 11 May 2022 09:26:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFCv2 09/10] x86/mm: Add userspace API to enable Linear Address
 Masking
Message-ID: <20220511072651.GV76023@worktop.programming.kicks-ass.net>
References: <20220511022751.65540-1-kirill.shutemov@linux.intel.com>
 <20220511022751.65540-11-kirill.shutemov@linux.intel.com>
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In-Reply-To: <20220511022751.65540-11-kirill.shutemov@linux.intel.com>
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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;
>  }
>