iommu/iova: Don't BUG on invalid PFNs

Unlike the other instances which represent a complete loss of
consistency within the rcache mechanism itself, or a fundamental
and obvious misconfiguration by an IOMMU driver, the BUG_ON() in
iova_magazine_free_pfns() can be provoked at more or less any time
in a "spooky action-at-a-distance" manner by any old device driver
passing nonsense to dma_unmap_*() which then propagates through to
queue_iova().

Not only is this well outside the IOVA layer's control, it's also
nowhere near fatal enough to justify panicking anyway - all that
really achieves is to make debugging the offending driver more
difficult. Let's simply WARN and otherwise ignore bogus PFNs.

Reported-by: Prakash Gupta <[email protected]>
Signed-off-by: Robin Murphy <[email protected]>
Reviewed-by: Prakash Gupta <[email protected]>
Link: https://lore.kernel.org/r/acbd2d092b42738a03a21b417ce64e27f8c91c86.1591103298.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <[email protected]>

Author: rmurphy-arm

drivers/iommu/iova.c

@@ -811,7 +811,9 @@ iova_magazine_free_pfns(struct iova_magazine *mag, struct iova_domain *iovad)
 	for (i = 0 ; i < mag->size; ++i) {
 		struct iova *iova = private_find_iova(iovad, mag->pfns[i]);
 
-		BUG_ON(!iova);
+		if (WARN_ON(!iova))
+			continue;
+
 		private_free_iova(iovad, iova);
 	}