iommu/io-pgtable-arm: Avoid dereferencing bogus PTEs

In the case of corrupted page tables, or when an invalid size is given,
__arm_lpae_unmap() may recurse beyond the maximum number of levels.
Unfortunately the detection of this error condition only happens *after*
calculating a nonsense offset from something which might not be a valid
table pointer and dereferencing that to see if it is a valid PTE.

Make things a little more robust by checking the level is valid before
doing anything which depends on it being so.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

drivers/iommu/io-pgtable-arm.c

@@ -486,11 +486,13 @@ static int __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
 	void *cookie = data->iop.cookie;
 	size_t blk_size = ARM_LPAE_BLOCK_SIZE(lvl, data);
 
+	/* Something went horribly wrong and we ran out of page table */
+	if (WARN_ON(lvl == ARM_LPAE_MAX_LEVELS))
+		return 0;
+
 	ptep += ARM_LPAE_LVL_IDX(iova, lvl, data);
 	pte = *ptep;
-
-	/* Something went horribly wrong and we ran out of page table */
-	if (WARN_ON(!pte || (lvl == ARM_LPAE_MAX_LEVELS)))
+	if (WARN_ON(!pte))
 		return 0;
 
 	/* If the size matches this level, we're in the right place */