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

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Date:	Fri, 11 Sep 2015 18:51:08 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Julien Grall <julien.grall@...rix.com>
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 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.

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