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

From	Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and structures
Date	2017-08-17 01:10 +0200
Message-ID	<uffBL-4VU-5@gated-at.bofh.it> (permalink)
References	(1 earlier) <uclNo-6X2-11@gated-at.bofh.it> <uegJC-w3-15@gated-at.bofh.it> <uegJC-w3-13@gated-at.bofh.it> <uet46-86q-21@gated-at.bofh.it> <uf5j4-6Uh-19@gated-at.bofh.it>
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Michael Ellerman [mpe@ellerman.id.au] wrote:
> Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> writes:
> 
> > Nicholas Piggin [npiggin@gmail.com] wrote:
> >> On Mon, 14 Aug 2017 15:21:48 +1000
> >> Michael Ellerman <mpe@ellerman.id.au> wrote:
> >> 
> >> > Sukadev Bhattiprolu <sukadev@linux.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

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Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields  and structures Nicholas Piggin <npiggin@gmail.com> - 2017-08-14 08:10 +0200
  Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and structures Michael Ellerman <mpe@ellerman.id.au> - 2017-08-14 13:10 +0200
  Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and  structures Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> - 2017-08-14 21:20 +0200
    Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and structures Michael Ellerman <mpe@ellerman.id.au> - 2017-08-16 14:10 +0200
      Re: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and  structures Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> - 2017-08-17 01:10 +0200

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