iommu/iova: Sort out rbtree limit_pfn handling

When walking the rbtree, the fact that iovad->start_pfn and limit_pfn
are both inclusive limits creates an ambiguity once limit_pfn reaches
the bottom of the address space and they overlap. Commit 5016bdb796b3
("iommu/iova: Fix underflow bug in __alloc_and_insert_iova_range") fixed
the worst side-effect of this, that of underflow wraparound leading to
bogus allocations, but the remaining fallout is that any attempt to
allocate start_pfn itself erroneously fails.

The cleanest way to resolve the ambiguity is to simply make limit_pfn an
exclusive limit when inside the guts of the rbtree. Since we're working
with PFNs, representing one past the top of the address space is always
possible without fear of overflow, and elsewhere it just makes life a
little more straightforward.

Reported-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/dma-iommu.c

@@ -314,7 +314,7 @@ int iommu_dma_init_domain(struct iommu_domain *domain, dma_addr_t base,
 		 * If we have devices with different DMA masks, move the free
 		 * area cache limit down for the benefit of the smaller one.
 		 */
-		iovad->dma_32bit_pfn = min(end_pfn, iovad->dma_32bit_pfn);
+		iovad->dma_32bit_pfn = min(end_pfn + 1, iovad->dma_32bit_pfn);
 
 		return 0;
 	}

drivers/iommu/iova.c

@@ -48,7 +48,7 @@ init_iova_domain(struct iova_domain *iovad, unsigned long granule,
 	iovad->cached32_node = NULL;
 	iovad->granule = granule;
 	iovad->start_pfn = start_pfn;
-	iovad->dma_32bit_pfn = pfn_32bit;
+	iovad->dma_32bit_pfn = pfn_32bit + 1;
 	init_iova_rcaches(iovad);
 }
 EXPORT_SYMBOL_GPL(init_iova_domain);
@@ -63,7 +63,7 @@ __get_cached_rbnode(struct iova_domain *iovad, unsigned long *limit_pfn)
 		struct rb_node *prev_node = rb_prev(iovad->cached32_node);
 		struct iova *curr_iova =
 			rb_entry(iovad->cached32_node, struct iova, node);
-		*limit_pfn = curr_iova->pfn_lo - 1;
+		*limit_pfn = curr_iova->pfn_lo;
 		return prev_node;
 	}
 }
@@ -135,7 +135,7 @@ iova_insert_rbtree(struct rb_root *root, struct iova *iova,
 static unsigned int
 iova_get_pad_size(unsigned int size, unsigned int limit_pfn)
 {
-	return (limit_pfn + 1 - size) & (__roundup_pow_of_two(size) - 1);
+	return (limit_pfn - size) & (__roundup_pow_of_two(size) - 1);
 }
 
 static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
@@ -155,18 +155,15 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
 	while (curr) {
 		struct iova *curr_iova = rb_entry(curr, struct iova, node);
 
-		if (limit_pfn < curr_iova->pfn_lo)
+		if (limit_pfn <= curr_iova->pfn_lo) {
 			goto move_left;
-		else if (limit_pfn < curr_iova->pfn_hi)
-			goto adjust_limit_pfn;
-		else {
+		} else if (limit_pfn > curr_iova->pfn_hi) {
 			if (size_aligned)
 				pad_size = iova_get_pad_size(size, limit_pfn);
-			if ((curr_iova->pfn_hi + size + pad_size) <= limit_pfn)
+			if ((curr_iova->pfn_hi + size + pad_size) < limit_pfn)
 				break;	/* found a free slot */
 		}
-adjust_limit_pfn:
-		limit_pfn = curr_iova->pfn_lo ? (curr_iova->pfn_lo - 1) : 0;
+		limit_pfn = curr_iova->pfn_lo;
 move_left:
 		prev = curr;
 		curr = rb_prev(curr);
@@ -182,7 +179,7 @@ move_left:
 	}
 
 	/* pfn_lo will point to size aligned address if size_aligned is set */
-	new->pfn_lo = limit_pfn - (size + pad_size) + 1;
+	new->pfn_lo = limit_pfn - (size + pad_size);
 	new->pfn_hi = new->pfn_lo + size - 1;
 
 	/* If we have 'prev', it's a valid place to start the insertion. */
@@ -269,7 +266,7 @@ alloc_iova(struct iova_domain *iovad, unsigned long size,
 	if (!new_iova)
 		return NULL;
 
-	ret = __alloc_and_insert_iova_range(iovad, size, limit_pfn,
+	ret = __alloc_and_insert_iova_range(iovad, size, limit_pfn + 1,
 			new_iova, size_aligned);
 
 	if (ret) {