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/xarray.c b/lib/xarray.c
index 6f47f63..7576446 100644
--- 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)
