iommu/s390: Fix memory corruption when using identity domain

zpci_get_iommu_ctrs() returns counter information to be reported as part
of device statistics; these counters are stored as part of the s390_domain.
The problem, however, is that the identity domain is not backed by an
s390_domain and so the conversion via to_s390_domain() yields a bad address
that is zero'd initially and read on-demand later via a sysfs read.
These counters aren't necessary for the identity domain; just return NULL
in this case.

This issue was discovered via KASAN with reports that look like:
BUG: KASAN: global-out-of-bounds in zpci_fmb_enable_device
when using the identity domain for a device on s390.

Cc: [email protected]
Fixes: 64af12c ("iommu/s390: implement iommu passthrough via identity domain")
Reported-by: Cam Miller <[email protected]>
Signed-off-by: Matthew Rosato <[email protected]>
Tested-by: Cam Miller <[email protected]>
Reviewed-by: Farhan Ali <[email protected]>
Reviewed-by: Niklas Schnelle <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Joerg Roedel <[email protected]>

Author: rosatomj

drivers/iommu/s390-iommu.c

@@ -1032,7 +1032,8 @@ struct zpci_iommu_ctrs *zpci_get_iommu_ctrs(struct zpci_dev *zdev)
 
 	lockdep_assert_held(&zdev->dom_lock);
 
-	if (zdev->s390_domain->type == IOMMU_DOMAIN_BLOCKED)
+	if (zdev->s390_domain->type == IOMMU_DOMAIN_BLOCKED ||
+	    zdev->s390_domain->type == IOMMU_DOMAIN_IDENTITY)
 		return NULL;
 
 	s390_domain = to_s390_domain(zdev->s390_domain);