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SubjectRe: [PATCH v6 01/17] powerpc/vas: Define macros, register fields and structures
Sukadev Bhattiprolu <> writes:

> Nicholas Piggin [] wrote:
>> On Mon, 14 Aug 2017 15:21:48 +1000
>> Michael Ellerman <> wrote:
>> > Sukadev Bhattiprolu <> 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.


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