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

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Message-ID: <55F2F517.6020105@citrix.com>
Date:	Fri, 11 Sep 2015 16:36:55 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Ian Campbell <ian.campbell@...rix.com>,
	<xen-devel@...ts.xenproject.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	Riku Voipio <riku.voipio@...aro.org>
Subject: Re: [PATCH] arm/xen: Enable user access to the kernel before issuing
 a privcmd call

On 11/09/15 16:25, Russell King - ARM Linux wrote:
> On Fri, Sep 11, 2015 at 03:56:38PM +0100, Julien Grall wrote:
>> Well, we can't assume that the function will be called with uaccess
>> disabled.
> 
> Please explain your reasoning.

I think I was confused about the usage of uaccess.
Thank you for the explanation.

> The reason copy_from_user() et.al. need to save and restore the DACR is
> because the DACR may be in one of two states on older ARM architectures.
> It may have set the kernel domain to 'manager' mode, to allow these
> accessors to work on kernel memory, or the kernel domain may be in
> 'client' mode, thereby preventing the accessors from touching kernel
> memory.
> 
> Unless the code path is reachable with the kernel domain in manager
> mode, (iow, a set_fs(KERNEL_DS) or set_fs(get_ds()) has been done) then
> it should be safe to use uaccess_disable/uaccess_enable.


I believe that our privcmd driver doesn't made any usage of set_fs(...),
so I will use the uaccess_{disable,enable} macro.

Regards,

-- 
Julien Grall
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