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: 6b24ca4a1a8d ("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>

diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index 8b1c318189ce801a0935133b4882556fede5bfd2..e77d4856442c3f750434e37819e13688d210048e 100644 (file)
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -1463,6 +1463,25 @@ unlock:
        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();

diff --git a/lib/xarray.c b/lib/xarray.c
index 6f47f6375808a7f54694bfaab8c0f8d2f68ff18e..757644617b9b8ff9ebad57508c3fca774ee51657 100644 (file)
--- a/lib/xarray.c
+++ b/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)