linux-kernel - Re: [PATCH v2] arm/xen: Enable user access to the kernel before issuing a privcmd call

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Message-ID: <55F31105.6070305@citrix.com>
Date:	Fri, 11 Sep 2015 18:36:05 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	<ian.campbell@...rix.com>, <stefano.stabellini@...citrix.com>,
	Riku Voipio <riku.voipio@...aro.org>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] arm/xen: Enable user access to the kernel before issuing
 a privcmd call

On 11/09/15 18:32, Julien Grall wrote:
> On 11/09/15 18:00, Russell King - ARM Linux wrote:
>> On Fri, Sep 11, 2015 at 05:25:59PM +0100, Julien Grall wrote:
>>> +	/*
>>> +	 * Privcmd calls are issued by the userspace. We need to allow the
>>> +	 * kernel to access the userspace memory before issuing the hypercall.
>>> +	 */
>>> +	uaccess_enable r4
>>> +
>>> +	/* r4 is loaded now as we use it as scratch register before */
>>>  	ldr r4, [sp, #4]
>>
>> As I mentioned in one of my previous mails, "ip" should be safe to use
>> here - it's a caller-corrupted register, just like r0-r3 and lr.  So,
>> you could do:
>>
>> 	ldr r4, [sp, #4]
>> +	uaccess_enable ip
> 
> The register ip (aka r12) is used to store the hypercall number. So we
> can't reuse it as scratch register.
> 
> The easiest one is r4.
> 
>>
>> which fractionally tightens the window.
>>
>> However, there's nothing actually wrong with your version - there's no
>> way we could've got this far with sp pointing at userspace.
>>
>> I'm happy with either version, so:
>>
>> Acked-by: Russell King <rmk+kernel@....linux.org.uk>
>>
>> How do you want to handle the patch?  I already have some other uaccess
>> fixes queued up to send to Linus before the merge window closes.

Forgot to answer to this bits. I was thinking to ask Stefano carrying
the patch in xentip. Although it won't go until rc1.

I don't mind if it's going earlier in Linux/master.

Regards,

-- 
Julien Grall
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