Re: [Xen-devel] [PATCH 04/19] xen: arm: provide and use a handle_raz_wi helper

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 02/04/2015 16:31, Ian Campbell wrote:
> On Thu, 2015-04-02 at 16:14 +0100, Julien Grall wrote:
> > Hi Ian,
> >
> > On 31/03/2015 11:07, Ian Campbell wrote:
> > > Reduces the use of goto in the trap handlers to none.
> > >
> > > Some explcitily 32-bit types become register_t here, but that's OK, on
> > s/explcitily/explicitly/
> >
> > > 32-bit they are 32-bit already and on 64-bit it is fine/harmless to
> > > set the larger register, a 32-bit guest won't see the top half in any
> > > case.
> > What about 32-bit userspace on 64-bit kernel? Are we sure that a guest
> > kernel won't only save the bottom half of the register?
> That would be fine, since the userspace couldn't see the top half anyway
> so not saving it doesn't hurt.
>
> In any case, the trap here has been talking from 32-bit mode and that is
> where we will return, so I'm not sure the guest kernel enters the
> picture, does it?
It's possible for the kernel to access only a part of the 64 bit 
registers and preserve the other part with a valid data to use later.
AFAICT, nothing prevent a guest to use the top half of the registers for 
his own purpose. It would only need to save/restore the bottom half of a 
64 bit registers.
By resetting the 64-bit register, we will corrupt the top half of the 
registers and potentially (if the use case is valid) crash the kernel.
Although I didn't say that I would write a such guest ;)

Regards,

--
Julien Grall

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