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

[Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>]

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@xxxxxxxxxxxxxxxx>
> >>
> >> 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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