linux-kernel - Re: [PATCH] arm64: Trap WFI executed in userspace

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Message-ID: <9af8bb9a-7c6c-2560-5965-118dfadf8141@arm.com>
Date:   Tue, 7 Aug 2018 11:24:34 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] arm64: Trap WFI executed in userspace

On 07/08/18 11:05, Dave Martin wrote:
> On Tue, Aug 07, 2018 at 10:33:26AM +0100, Marc Zyngier wrote:
>> It recently came to light that userspace can execute WFI, and that
>> the arm64 kernel doesn trap this event. This sounds rather benign,
>> but the kernel should decide when it wants to wait for an interrupt,
>> and not userspace.
>>
>> Let's trap WFI and treat it as a way to yield the CPU to another
>> process.
> 
> This doesn't amount to a justification.
> 
> If the power controller is unexpectedly left in a bad state so that
> WFI will do something nasty to a cpu that may enter userspace, then we
> probably have bigger problems.
> 
> So, maybe it really is pretty harmless to let userspace execute this.

Or not. It is also a very good way for userspace to find out when an
interrupt gets delivered and start doing all kind of probing on the
kernel. The least the userspace knows about that, the better I feel.

> I can't think of a legitimate reason for userspace to execute WFI
> however.  Userspace doesn't have interrupts under Linux, so it makes
> no sense to wait for one.
> 
> Have we seen anybody using WFI in userspace?  It may be cleaner to
> map this to SIGILL rather than be permissive and regret it later.

I couldn't find any user, and I'm happy to just send userspace to hell
in that case. But it could also been said that since it was never
prevented, it is a de-facto ABI.

	M.
-- 
Jazz is not dead. It just smells funny...