* Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [230310 09:09]:
> There is a concurrency bug that may cause the wrong value to be loaded
> when a CPU is modifying the maple tree.
>
> CPU1:
> mtree_insert_range()
> mas_insert()
> mas_store_root()
> ...
> mas_root_expand()
> ...
> rcu_assign_pointer(mas->tree->ma_root, mte_mk_root(mas->node));
> ma_set_meta(node, maple_leaf_64, 0, slot); <---IP
>
> CPU2:
> mtree_load()
> mtree_lookup_walk()
> ma_data_end();
>
> When CPU1 is about to execute the instruction pointed to by IP,
> the ma_data_end() executed by CPU2 may return the wrong end position,
> which will cause the value loaded by mtree_load() to be wrong.
>
> An example of triggering the bug:
>
> Add mdelay(100) between rcu_assign_pointer() and ma_set_meta() in
> mas_root_expand().
>
> static DEFINE_MTREE(tree);
> int work(void *p) {
> unsigned long val;
> for (int i = 0 ; i< 30; ++i) {
> val = (unsigned long)mtree_load(&tree, 8);
> mdelay(5);
> pr_info("%lu",val);
> }
> return 0;
> }
>
> mt_init_flags(&tree, MT_FLAGS_USE_RCU);
> mtree_insert(&tree, 0, (void*)12345, GFP_KERNEL);
> run_thread(work)
> mtree_insert(&tree, 1, (void*)56789, GFP_KERNEL);
>
> In RCU mode, mtree_load() should always return the value before or after
> the data structure is modified, and in this example mtree_load(&tree, 8)
> may return 56789 which is not expected, it should always return NULL.
> Fix it by put ma_set_meta() before rcu_assign_pointer().
Are you able to write a test case for this issue? I understand it's a
race so it may be difficult to catch.
>
> Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
> lib/maple_tree.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 4d15202a0692..de43ff19da72 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -3635,10 +3635,9 @@ static inline int mas_root_expand(struct ma_state *mas, void *entry)
> slot++;
> mas->depth = 1;
> mas_set_height(mas);
> -
> + ma_set_meta(node, maple_leaf_64, 0, slot);
> /* swap the new root into the tree */
> rcu_assign_pointer(mas->tree->ma_root, mte_mk_root(mas->node));
> - ma_set_meta(node, maple_leaf_64, 0, slot);
> return slot;
> }
>
> --
> 2.20.1
>
