Re: [PATCH RFC v4] xarray: fix index jumping backwards in xas_find()

[Krystian Kaniewski <krystianmkaniewski@gmail.com>]

#syz upstream

On 8/28/2026 10:23 AM, syzbot wrote:
> A bug in the XArray iterator xas_find() causes the iterator's index
> (xas->xa_index) to jump backwards when iterating over a multi-index entry
> (like a THP) that resides in a non-leaf node and is concurrently split.
>
> When iterating over a multi-index entry in a non-leaf node, xas_load() sets
> xas->xa_offset to the base offset of the entry, but leaves xas->xa_index at
> the requested index. When the caller subsequently wants to advance to the
> next entry, xas_find() is called. xas_find() attempts to synchronize
> xas->xa_offset with xas->xa_index before advancing. However, the fixup
> logic was incorrectly restricted to leaf nodes (!xas->xa_node->shift).
> Because the THP resides in a non-leaf node, the fixup is skipped.
>
> As a result, xas_find() simply increments xas->xa_offset and recalculates
> xas->xa_index based on this new offset. This causes xas->xa_index to jump
> backwards. If the THP was concurrently split, the entry at the new offset
> is a node pointer, so xas_find() descends into it and returns the folio at
> the backwards index. The caller (filemap_map_pages()) then calculates the
> PTE pointer based on this backwards index, resulting in an invalid memory
> access such as an out-of-bounds read or use-after-free on a page-table page
> freed via tlb_remove_table_rcu().
>
> To fix this, check if xas->xa_offset matches get_offset(xas->xa_index,
> xas->xa_node). If it does not and the node is a non-leaf node, set
> xas->xa_offset to get_offset(xas->xa_index, xas->xa_node) before advancing.
> Also add test cases in test_xarray to verify xas_find() behavior when
> iterating over and splitting multi-index entries.
>
> Fixes: b803b42823d0 ("xarray: Add XArray iterators")
> Assisted-by: Gemini:gemini-3.7-flash syzbot
> Reported-by: syzbot+b72767277f29b6407083@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b72767277f29b6407083
> Link: https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
> To: "Andrew Morton" <akpm@linux-foundation.org>
> To: <linux-fsdevel@vger.kernel.org>
> To: <linux-mm@kvack.org>
> To: "Matthew Wilcox" <willy@infradead.org>
> Cc: <linux-kernel@vger.kernel.org>
>
> ---
> v4:
> - Removed verbatim KASAN crash report excerpt from the commit description.
>
> v3:
> - Renamed xas_split to split and passed xa_mk_index(0) to xas_split_alloc() and xas_split() in test_xarray.
> - Updated commit description to clarify that the use-after-free occurs on a page-table page freed via tlb_remove_table_rcu().
> - Changed subject prefix to lowercase 'xarray:'.
> - Corrected the quoted crash report in the commit description.
> https://lore.kernel.org/all/e2842b55-5d7f-47c1-bff1-28162cc1578d@mail.kernel.org/T/
>
> v2:
> - Corrected offset synchronization for non-leaf nodes to use get_offset(xas->xa_index, xas->xa_node).
> - Added check_multi_find_4() self-test in lib/test_xarray.c to test multi-index entry lookups and concurrent splitting.
> - Updated the commit description to include the KASAN slab-use-after-free report.
> https://lore.kernel.org/all/fc978b5d-df9f-4a06-89f5-d0fed8939e42@mail.kernel.org/T/
>
> v1:
> https://lore.kernel.org/all/e62bb2ac-6ecf-41a2-823f-c13547d5db78@mail.kernel.org/T/
> ---
> diff --git a/lib/test_xarray.c b/lib/test_xarray.c
> index 5ca0aefee..cda7245e4 100644
> --- a/lib/test_xarray.c
> +++ b/lib/test_xarray.c
> @@ -1247,6 +1247,67 @@ static noinline void check_multi_find_3(struct xarray *xa)
>   	}
>   }
>   
> +static noinline void check_multi_find_4(struct xarray *xa)
> +{
> +#ifdef CONFIG_XARRAY_MULTI
> +	XA_STATE(xas, xa, 100);
> +	XA_STATE_ORDER(split, xa, 0, 0);
> +	void *entry;
> +	unsigned long i;
> +
> +	/* (1) Order-7 entry (0-127) with adjacent entry at 128, starting at 100 */
> +	xa_store_order(xa, 0, 7, xa_mk_index(0), GFP_KERNEL);
> +	XA_BUG_ON(xa, xa_store_index(xa, 128, GFP_KERNEL) != NULL);
> +
> +	rcu_read_lock();
> +	entry = xas_find(&xas, ULONG_MAX);
> +	XA_BUG_ON(xa, entry != xa_mk_index(0));
> +	XA_BUG_ON(xa, xas.xa_index != 100);
> +
> +	entry = xas_find(&xas, ULONG_MAX);
> +	XA_BUG_ON(xa, entry != xa_mk_index(128));
> +	XA_BUG_ON(xa, xas.xa_index != 128);
> +
> +	entry = xas_find(&xas, ULONG_MAX);
> +	XA_BUG_ON(xa, entry != NULL);
> +	rcu_read_unlock();
> +
> +	xa_erase_index(xa, 128);
> +	xa_erase_index(xa, 0);
> +	XA_BUG_ON(xa, !xa_empty(xa));
> +
> +	/* (2) Splitting a multi-index entry after a lookup begins inside it */
> +	xa_store_order(xa, 0, 7, xa_mk_index(0), GFP_KERNEL);
> +	XA_BUG_ON(xa, xa_store_index(xa, 128, GFP_KERNEL) != NULL);
> +
> +	xas_set(&xas, 100);
> +	rcu_read_lock();
> +	entry = xas_find(&xas, ULONG_MAX);
> +	XA_BUG_ON(xa, entry != xa_mk_index(0));
> +	XA_BUG_ON(xa, xas.xa_index != 100);
> +	rcu_read_unlock();
> +
> +	xas_split_alloc(&split, xa_mk_index(0), 7, GFP_KERNEL);
> +	xas_lock(&split);
> +	xas_split(&split, xa_mk_index(0), 7);
> +	for (i = 0; i < 128; i++)
> +		__xa_store(xa, i, xa_mk_index(i), 0);
> +	xas_unlock(&split);
> +
> +	rcu_read_lock();
> +	entry = xas_find(&xas, ULONG_MAX);
> +	XA_BUG_ON(xa, entry != xa_mk_index(128));
> +	XA_BUG_ON(xa, xas.xa_index != 128);
> +
> +	entry = xas_find(&xas, ULONG_MAX);
> +	XA_BUG_ON(xa, entry != NULL);
> +	rcu_read_unlock();
> +
> +	xa_destroy(xa);
> +	XA_BUG_ON(xa, !xa_empty(xa));
> +#endif
> +}
> +
>   static noinline void check_find_1(struct xarray *xa)
>   {
>   	unsigned long i, j, k;
> @@ -1370,6 +1431,7 @@ static noinline void check_find(struct xarray *xa)
>   		check_multi_find_1(xa, i);
>   	check_multi_find_2(xa);
>   	check_multi_find_3(xa);
> +	check_multi_find_4(xa);
>   }
>   
>   /* See find_swap_entry() in mm/shmem.c */
> diff --git a/lib/xarray.c b/lib/xarray.c
> index 9a8b49165..980324d68 100644
> --- a/lib/xarray.c
> +++ b/lib/xarray.c
> @@ -1406,9 +1406,11 @@ void *xas_find(struct xa_state *xas, unsigned long max)
>   		entry = xas_load(xas);
>   		if (entry || xas_not_node(xas->xa_node))
>   			return entry;
> -	} else if (!xas->xa_node->shift &&
> -		    xas->xa_offset != (xas->xa_index & XA_CHUNK_MASK)) {
> -		xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1;
> +	} else if (xas->xa_offset != get_offset(xas->xa_index, xas->xa_node)) {
> +		if (!xas->xa_node->shift)
> +			xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1;
> +		else
> +			xas->xa_offset = get_offset(xas->xa_index, xas->xa_node);
>   	}
>   
>   	xas_next_offset(xas);
>
>
> base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f