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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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.
> 
>>      __HVC(XEN_IMM)
>> +
>> +    /*
>> +     * Disable userspace access from kernel. This is fine to do it
>> +     * unconditionally as no set_fs(KERNEL_DS)/set_fs(get_ds()) is
>> +     * called before.
>> +     */
>> +    uaccess_disable r4
>> +
>>      ldm sp!, {r4}
>>      ret lr
>>  ENDPROC(privcmd_call);
>> -- 
>> 2.1.4
>>
> 


-- 
Julien Grall

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