maple_tree: fix allocation in mas_sparse_area()

In the case of reverse allocation, mas->index and mas->last do not point
to the correct allocation range, which will cause users to get incorrect
allocation results, so fix it.  If the user does not use it in a specific
way, this bug will not be triggered.

This is a bug, but only VMA uses it now, the way VMA is used now will
not trigger it.  There is a possibility that a user will trigger it in
the future.

Also re-check whether the size is still satisfied after the lower bound
was increased, which is a corner case and is incorrect in previous
versions.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 54a611b ("Maple Tree: add new data structure")
Signed-off-by: Peng Zhang <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

lib/maple_tree.c

@@ -5250,25 +5250,28 @@ static inline void mas_fill_gap(struct ma_state *mas, void *entry,
  * @size: The size of the gap
  * @fwd: Searching forward or back
  */
-static inline void mas_sparse_area(struct ma_state *mas, unsigned long min,
+static inline int mas_sparse_area(struct ma_state *mas, unsigned long min,
 				unsigned long max, unsigned long size, bool fwd)
 {
-	unsigned long start = 0;
-
-	if (!unlikely(mas_is_none(mas)))
-		start++;
+	if (!unlikely(mas_is_none(mas)) && min == 0) {
+		min++;
+		/*
+		 * At this time, min is increased, we need to recheck whether
+		 * the size is satisfied.
+		 */
+		if (min > max || max - min + 1 < size)
+			return -EBUSY;
+	}
 	/* mas_is_ptr */
 
-	if (start < min)
-		start = min;
-
 	if (fwd) {
-		mas->index = start;
-		mas->last = start + size - 1;
-		return;
+		mas->index = min;
+		mas->last = min + size - 1;
+	} else {
+		mas->last = max;
+		mas->index = max - size + 1;
 	}
-
-	mas->index = max;
+	return 0;
 }
 
 /*
@@ -5297,10 +5300,8 @@ int mas_empty_area(struct ma_state *mas, unsigned long min,
 		return -EBUSY;
 
 	/* Empty set */
-	if (mas_is_none(mas) || mas_is_ptr(mas)) {
-		mas_sparse_area(mas, min, max, size, true);
-		return 0;
-	}
+	if (mas_is_none(mas) || mas_is_ptr(mas))
+		return mas_sparse_area(mas, min, max, size, true);
 
 	/* The start of the window can only be within these values */
 	mas->index = min;
@@ -5356,10 +5357,8 @@ int mas_empty_area_rev(struct ma_state *mas, unsigned long min,
 	}
 
 	/* Empty set. */
-	if (mas_is_none(mas) || mas_is_ptr(mas)) {
-		mas_sparse_area(mas, min, max, size, false);
-		return 0;
-	}
+	if (mas_is_none(mas) || mas_is_ptr(mas))
+		return mas_sparse_area(mas, min, max, size, false);
 
 	/* The start of the window can only be within these values. */
 	mas->index = min;