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

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Message-ID: <alpine.DEB.2.02.1509111856020.2672@kaball.uk.xensource.com>
Date:	Fri, 11 Sep 2015 18:56:50 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Julien Grall <julien.grall@...rix.com>, <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 Fri, 11 Sep 2015, Russell King - ARM Linux wrote:
> On Fri, Sep 11, 2015 at 06:36:05PM +0100, Julien Grall wrote:
> > 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.
> 
> Thanks, I've applied your patch as-is now.

That's fine by me, the patch looks good.

Thanks,

Stefano
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