RE: [for-4.15][PATCH v2 3/5] xen/iommu: iommu_map: Don't crash the domain if it is dying

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 09 February 2021 15:28
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: hongyxia@xxxxxxxxxxxx; iwj@xxxxxxxxxxxxxx; Julien Grall 
> <jgrall@xxxxxxxxxx>; Jan Beulich
> <jbeulich@xxxxxxxx>; Paul Durrant <paul@xxxxxxx>
> Subject: [for-4.15][PATCH v2 3/5] xen/iommu: iommu_map: Don't crash the 
> domain if it is dying
> 
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> It is a bit pointless to crash a domain that is already dying. This will
> become more an annoyance with a follow-up change where page-table
> allocation will be forbidden when the domain is dying.
> 
> Security wise, there is no change as the devices would still have access
> to the IOMMU page-tables even if the domain has crashed until Xen
> start to relinquish the resources.
> 
> For x86, we rely on dom_iommu(d)->arch.mapping.lock to ensure
> d->is_dying is correctly observed (a follow-up patch will held it in the

s/held/hold

> relinquish path).
> 
> For Arm, there is still a small race possible. But there is so far no
> failure specific to a domain dying.
> 
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ---
> 
> This was spotted when trying to destroy IOREQ servers while the domain
> is dying. The code will try to add the entry back in the P2M and
> therefore update the P2M (see arch_ioreq_server_disable() ->
> hvm_add_ioreq_gfn()).
> 
> It should be possible to skip the mappin in hvm_add_ioreq_gfn(), however

s/mappin/mapping

> I didn't try a patch yet because checking d->is_dying can be racy (I
> can't find a proper lock).
> 
> Changes in v2:
>     - Patch added
> ---
>  xen/drivers/passthrough/iommu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/xen/drivers/passthrough/iommu.c b/xen/drivers/passthrough/iommu.c
> index 879d238bcd31..75419f20f76d 100644
> --- a/xen/drivers/passthrough/iommu.c
> +++ b/xen/drivers/passthrough/iommu.c
> @@ -272,7 +272,7 @@ int iommu_map(struct domain *d, dfn_t dfn, mfn_t mfn,
>                              flush_flags) )
>                  continue;
> 
> -        if ( !is_hardware_domain(d) )
> +        if ( !is_hardware_domain(d) && !d->is_dying )
>              domain_crash(d);

Would it make more sense to check is_dying inside domain_crash() (and turn it 
into a no-op in that case)?

  Paul

> 
>          break;
> --
> 2.17.1