Re: [PATCH v2] xsplice: Don't perform multiple operations on same payload once work is scheduled.

[Wei Liu <wei.liu2@citrix.com>]

On Fri, Apr 29, 2016 at 05:42:35AM -0400, Konrad Rzeszutek Wilk wrote:
> Currently it is possible to:
> 
> 1)  xc_xsplice_apply()
>      \-> xsplice_action
> 	spin_lock(payload_lock)
>              \- schedule_work()
>         spin_unlock(payload_lock);
> 
> 2)  xc_xsplice_unload()
>      \-> xsplice_action
> 	spin_lock(payload_lock)
>              free_payload(data);
>         spin_unlock(payload_lock);
> 
> .. all CPUs are quiesced.
> 
> 3) check_for_xsplice_work()
>      \-> apply_payload
>         \-> arch_xsplice_apply_jmp
> 		BOOM
> 
> The reason is that state is in 'CHECKED' which changes to 'APPLIED'
> once check_for_xsplice_work finishes. So we have a race between 1) -> 3)
> where one can manipulate the payload.
> 
> To guard against this we add a check in xsplice_action to not allow
> any actions if schedule_work has been called for this specific payload.
> 
> The function 'is_work_scheduled' checks xsplice_work which is safe as:
>  - The ->do_work changes to 1 under the payload_lock (which we also hold).
>  - The ->do_work changes to 0 when all CPUs are quisced and IRQs have
>    been disabled.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Reported-and-Tested-by: Roger Pau Monné <roger.pau@citrix.com>
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

Release-acked-by: Wei Liu <wei.liu2@citrix.com>

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