linux-kernel - Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and structures

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Date:   Wed, 16 Aug 2017 22:07:38 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        mikey@...ling.org, stewart@...ux.vnet.ibm.com, apopple@....ibm.com,
        hbabu@...ibm.com, oohall@...il.com, linuxppc-dev@...abs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and structures

Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com> writes:

> Nicholas Piggin [npiggin@...il.com] wrote:
>> On Mon, 14 Aug 2017 15:21:48 +1000
>> Michael Ellerman <mpe@...erman.id.au> wrote:
>> 
>> > Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com> writes:
>> 
>> > >  arch/powerpc/include/asm/vas.h       |  35 ++++
>> > >  arch/powerpc/include/uapi/asm/vas.h  |  25 +++  
>> > 
>> > I thought we weren't exposing VAS to userspace yet?
>> > 
>> > If we are then we need to get things straight WRT copy/paste abort.
...
>
> In the FTW case, there is no data transfer from user space to the hardware.
> i.e the copy/paste submit a NULL CRB and hardware will be configured (see
> ->fifo_disable setting in winctx) to ignore any data they specify in the CRB.

I thought the copy did copy a cacheline, but then the paste to the VAS
window just ignores the contents, and doesn't allow userspace to get the
content in any way?

Which means we have two thirds of a covert channel, ie. something can be
copied into the copy buffer by one process, and then a second process
can paste it, but because it can only paste to foreign memory, and the
only foreign memory it can get is a VAS FTW window, it can't actually
see the content of the copy buffer.

> Would we be able to allow copy/paste from user space in that case?

Yeah I think so, but it is all a bit fragile.

cheers