From: Matthew Wilcox (Oracle) willy@infradead.org
commit 3e3c658055c002900982513e289398a1aad4a488 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- lib/test_xarray.c | 22 ++++++++++++++++++++++ lib/xarray.c | 2 ++ 2 files changed, 24 insertions(+)
--- a/lib/test_xarray.c +++ b/lib/test_xarray.c @@ -1438,6 +1438,25 @@ 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; @@ -1465,6 +1484,9 @@ static noinline void check_create_range( 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(); --- a/lib/xarray.c +++ b/lib/xarray.c @@ -722,6 +722,8 @@ void xas_create_range(struct xa_state *x
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)