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

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Message-ID: <55F2EBA6.6060008@citrix.com>
Date:	Fri, 11 Sep 2015 15:56:38 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Ian Campbell <ian.campbell@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	"Riku Voipio" <riku.voipio@...aro.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH] arm/xen: Enable user access to the kernel before issuing
 a privcmd call

On 11/09/15 15:55, Ian Campbell wrote:
> On Fri, 2015-09-11 at 15:45 +0100, Julien Grall wrote:
>> On 11/09/15 15:29, Ian Campbell wrote:
>>> On Fri, 2015-09-11 at 15:16 +0100, Julien Grall wrote:
>>>> When Xen is copyin data to/from the guest it will check if the kernel
>>>
>>> "copying"
>>>
>>>> has the right to do the access. If not, the hypercall will return an
>>>> error.
>>>>
>>>> After the commit a5e090acbf545c0a3b04080f8a488b17ec41fe02 "ARM:
>>>> software-based priviledged-no-access support", the kernel can't
>>>> access
>>>
>>> "privileged"
>>>
>>>> anymore the user space by default. This will result to fail on every
>>>
>>> "any more" (or "any longer")
>>>
>>>> hypercall made by the userspace (i.e via privcmd).
>>>>
>>>> We have to enable the userspace access and then restore the correct
>>>> permission everytime the privmcd is used to made an hypercall.
>>>
>>> "every time" and "privcmd"
>>>
>>>>  HYPERCALL1(tmem_op);
>>>>  HYPERCALL2(multicall);
>>>>  
>>>> -ENTRY(privcmd_call)
>>>> +ENTRY(__privcmd_call)
>>>
>>> arch/arm/include/asm/assembler.h seems to contain uaccess_* macros
>>> which
>>> could be used right here directly I think? That would be preferable to
>>> wrapping I think.
>>
>> Looking to the uaccess_save macro:
> 
> I was thinking more about uaccess_enable/disable.

Well, we can't assume that the function will be called with uaccess
disabled.  So we have to save the state and restore it after issuing the
hypercall.

Regards,

-- 
Julien Grall
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