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

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Message-ID: <55F3102B.3090309@citrix.com>
Date:	Fri, 11 Sep 2015 18:32:27 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	<xen-devel@...ts.xenproject.org>,
	<linux-arm-kernel@...ts.infradead.org>, <ian.campbell@...rix.com>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	Riku Voipio <riku.voipio@...aro.org>
Subject: Re: [PATCH v2] arm/xen: Enable user access to the kernel before issuing
 a privcmd call

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.
> 
>>  	__HVC(XEN_IMM)
>> +
>> +	/*
>> +	 * Disable userspace access from kernel. This is fine to do it
>> +	 * unconditionally as no set_fs(KERNEL_DS)/set_fs(get_ds()) is
>> +	 * called before.
>> +	 */
>> +	uaccess_disable r4
>> +
>>  	ldm sp!, {r4}
>>  	ret lr
>>  ENDPROC(privcmd_call);
>> -- 
>> 2.1.4
>>
> 


-- 
Julien Grall
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