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maple_tree: fix a potential concurrency bug in RCU mode

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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().

Link: https://lkml.kernel.org/r/20230314124203.91572-4-zhangpeng.00@bytedance.com
Fixes: 54a611b605 ("Maple Tree: add new data structure")
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Peng Zhang 2023-03-14 20:42:03 +08:00 committed by Andrew Morton
parent ec07967d75
commit c45ea315a6
1 changed files with 1 additions and 2 deletions
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3
lib/maple_tree.c
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@ -3725,10 +3725,9 @@ 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;
}