XArray: Fix xas_create_range() when multi-order entry present

If there is already an entry present that is of order >= XA_CHUNK_SHIFT
when we call xas_create_range(), xas_create_range() will misinterpret
that entry as a node and dereference xa_node->parent, generally leading
to a crash that looks something like this:

general protection fault, probably for non-canonical address 0xdffffc0000000001:
0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 0 PID: 32 Comm: khugepaged Not tainted 5.17.0-rc8-syzkaller-00003-g56e337f2cf13 #0
RIP: 0010:xa_parent_locked include/linux/xarray.h:1207 [inline]
RIP: 0010:xas_create_range+0x2d9/0x6e0 lib/xarray.c:725

It's deterministically reproducable once you know what the problem is,
but producing it in a live kernel requires khugepaged to hit a race.
While the problem has been present since xas_create_range() was
introduced, I'm not aware of a way to hit it before the page cache was
converted to use multi-index entries.

Fixes: 6b24ca4 ("mm: Use multi-index entries in the page cache")
Reported-by: syzbot+0d2b0bf32ca5cfd09f2e@syzkaller.appspotmail.com
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

lib/test_xarray.c

@@ -1463,6 +1463,25 @@ static noinline void check_create_range_4(struct xarray *xa,
 	XA_BUG_ON(xa, !xa_empty(xa));
 }
 
+static noinline void check_create_range_5(struct xarray *xa,
+		unsigned long index, unsigned int order)
+{
+	XA_STATE_ORDER(xas, xa, index, order);
+	unsigned int i;
+
+	xa_store_order(xa, index, order, xa_mk_index(index), GFP_KERNEL);
+
+	for (i = 0; i < order + 10; i++) {
+		do {
+			xas_lock(&xas);
+			xas_create_range(&xas);
+			xas_unlock(&xas);
+		} while (xas_nomem(&xas, GFP_KERNEL));
+	}
+
+	xa_destroy(xa);
+}
+
 static noinline void check_create_range(struct xarray *xa)
 {
 	unsigned int order;
@@ -1490,6 +1509,9 @@ static noinline void check_create_range(struct xarray *xa)
 		check_create_range_4(xa, (3U << order) + 1, order);
 		check_create_range_4(xa, (3U << order) - 1, order);
 		check_create_range_4(xa, (1U << 24) + 1, order);
+
+		check_create_range_5(xa, 0, order);
+		check_create_range_5(xa, (1U << order), order);
 	}
 
 	check_create_range_3();

lib/xarray.c

@@ -722,6 +722,8 @@ void xas_create_range(struct xa_state *xas)
 
 		for (;;) {
 			struct xa_node *node = xas->xa_node;
+			if (node->shift >= shift)
+				break;
 			xas->xa_node = xa_parent_locked(xas->xa, node);
 			xas->xa_offset = node->offset - 1;
 			if (node->offset != 0)