RE: [PATCH V4 13/24] xen/ioreq: Use guest_cmpxchg64() instead of cmpxchg()

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of 
> Oleksandr Tyshchenko
> Sent: 12 January 2021 21:52
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>; Paul Durrant 
> <paul@xxxxxxx>; Julien Grall
> <julien@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Julien Grall 
> <julien.grall@xxxxxxx>
> Subject: [PATCH V4 13/24] xen/ioreq: Use guest_cmpxchg64() instead of 
> cmpxchg()
> 
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> 
> The cmpxchg() in ioreq_send_buffered() operates on memory shared
> with the emulator domain (and the target domain if the legacy
> interface is used).
> 
> In order to be on the safe side we need to switch
> to guest_cmpxchg64() to prevent a domain to DoS Xen on Arm.
> 
> As there is no plan to support the legacy interface on Arm,
> we will have a page to be mapped in a single domain at the time,
> so we can use s->emulator in guest_cmpxchg64() safely.
> 
> Thankfully the only user of the legacy interface is x86 so far
> and there is not concern regarding the atomics operations.
> 
> Please note, that the legacy interface *must* not be used on Arm
> without revisiting the code.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> [On Arm only]
> Tested-by: Wei Chen <Wei.Chen@xxxxxxx>

Reviewed-by: Paul Durrant <paul@xxxxxxx>