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

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Message-Id: <20170816230742.GE31897@us.ibm.com>
Date:   Wed, 16 Aug 2017 16:07:42 -0700
From:   Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        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

Michael Ellerman [mpe@...erman.id.au] wrote:
> 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.

Sorry, that was focussed on the paste side.

> > 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?

Yes, you are right. The copy instruction does read the CRB into its copy-
buffer but for the FTW, VAS ignores the copy-buffer contents on paste.
So, the CRB may be zeroed, but must be a valid buffer.

> 
> 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