[PATCH v3 0/2] maple_tree: fix maple_range_64 crashes in RCU mode
From: Dimitris Charisis
Date: Mon Oct 05 2026 - 08:58:17 EST
The patch series fixes two RCU destroy path issues on maple_range_64
nodes. Both arising from the fact that a maple_range_64 node repurposes
the last slot to hold metadata when the node is not full. Patch 1
addresses the issues, and patch 2 adds a test used to catch them.
Currently there is no in-tree user of maple_range_64 nodes in RCU mode.
All in-tree users of MT_FLAGS_USE_RCU also set MT_FLAGS_ALLOC_RANGE
which makes the internal nodes maple_arange_64.
Below is the minimal reproducer used during debugging:
static DEFINE_MTREE(tree);
static int __init maple_fix_init(void)
{
unsigned long i, j;
for (i = 10; i < 4000; i++) {
mt_init_flags(&tree, MT_FLAGS_USE_RCU);
for (j = 0; j < i; j++) {
mtree_insert_range(&tree, j * 10, j * 10 + 9,
xa_mk_value(j), GFP_KERNEL);
}
mtree_destroy(&tree);
rcu_barrier();
}
return 0;
}
The splat for the first issue addressed by the first patch is shown
below. mt_clear_meta() runs after the slots have been rewritten with raw
pointers by mte_dead_leaves(), so it cannot recognize a valid last
pointer because the type information is stripped. On my 64-bit LE
machine, it clears bytes 248 and 249 of the maple_range_64 which are the
two least significant bytes of the last pointer. The lowest byte is
already zero, since nodes are 256-byte aligned, but zeroing byte 249
corrupts the pointer.
KASAN catches it as a double-free, because that corrupted pointer
happens to land on a node that was freed previously. The splat appears
at 226 insertions in the reproducer. This is when the tree has a full
root node on a 64-bit build.
==================================================================
BUG: KASAN: double-free in mt_free_walk+0x138/0x3a0
Free of addr ffff0000f3d40000 by task swapper/2/0
[...]
Call trace:
[...]
kmem_cache_free_bulk+0x5b8/0xc18
mt_free_walk+0x138/0x3a0
rcu_core+0x5d0/0x1638
rcu_core_si+0x18/0x30
[...]
Allocated by task 1218:
[...]
kmem_cache_alloc_from_sheaf_noprof+0xa8/0x2f0
dst_setup+0x210/0x630
mas_wr_split+0x568/0x26e0
mas_wr_store_entry+0x98c/0x1de0
mas_insert.isra.0+0x3d0/0x5d8
mtree_insert_range+0xec/0x1c8
maple_fix_init+0x94/0xff8 [maple_fix_obj]
[...]
Freed by task 0:
[...]
__kasan_slab_free+0x88/0xb8
__rcu_free_sheaf_prepare+0x94/0x360
rcu_free_sheaf+0x30/0x138
rcu_core+0x5d0/0x1638
rcu_core_si+0x18/0x30
[...]
The buggy address belongs to the object at ffff0000f3d40000
which belongs to the cache maple_node of size 256
The buggy address is located 0 bytes inside of
256-byte region [ffff0000f3d40000, ffff0000f3d40100)
[...]
Memory state around the buggy address:
ffff0000f3d3ff00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff0000f3d3ff80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff0000f3d40000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff0000f3d40080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff0000f3d40100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================
The second issue fixed in the first patch is in the mt_free_walk().
mt_free_walk() descends by checking if the corresponding slot[offset]
entry is non-zero. In a maple_range_64 node that has
MAPLE_RANGE64_SLOTS-1 valid pointers to child nodes, all slots will be
non-zero, since the last slot holds the metadata. Thus, mte_dead_walk()
dereferences the metadata as if it were a node and hits the
null-ptr-deref as shown in the splat below.
After fixing the first issue, the reproducer reaches this case at 3166
insertions. The affected node is the root on a tree with height 3 and it
has 15 valid child nodes on a 64-bit machine. The first fix does not
introduce this bug, but it exposes it for the root node because it no
longer clears the root metadata. On an unmodified kernel, the same bug
exists and is reachable when such a maple_range_64 node appears below
the root, where mt_clear_meta() is not called.
Second KASAN splat:
Unable to handle kernel paging request at virtual address dfff800000000006
KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
[...]
Call trace:
mte_dead_walk+0x78/0x140 (P)
mt_free_walk+0x1f8/0x3a0
rcu_core+0x5d0/0x1638
rcu_core_si+0x18/0x30
[..]
Both splats are from an arm64 QEMU guest. The test_maple_tree and the
userspace suite pass after each patch in the series.
---
Note for reviewers:
* Before commit 790e1fa86b34 ("maple_tree: add RCU lock checking to rcu
callback functions") mas_clear_meta() was called while descending the
tree, so metadata was cleared on every node. That commit removed the
call from the descend path leaving only the final call for the root
node, and renamed it to mt_clear_meta().
An alternative fix would be to restore metadata clearing for every
node and fix mt_clear_meta() for full maple_range_64 nodes. I
preferred removing the clearing and use slot_len to identify the valid
slots directly.
Signed-off-by: Dimitris Charisis <dchar@xxxxxxxxxxxxxxxxx>
---
Changes in v3:
- Combine both fixes into one patch, as suggested by Liam, to avoid
leaving an intermediate commit where removing mt_clear_meta() exposes
the second bug at the root node.
- Combine both regression tests into the next patch.
- Link to v2: https://lore.kernel.org/r/20260916-fix-maple-tree-range64-rcu-v2-0-0b09de37eb69@xxxxxxxxxxxxxxxxx
Changes in v2:
- Add one test after each fix.
- Reword the cover letter.
- Link to v1: https://lore.kernel.org/r/20260913-fix-maple-tree-range64-rcu-v1-0-31a130bb8cbf@xxxxxxxxxxxxxxxxx
---
Dimitris Charisis (2):
maple_tree: fix maple_range_64 crashes in RCU mode
test_maple_tree: test maple_range_64 node in RCU mode
lib/maple_tree.c | 43 +------------------------------------------
lib/test_maple_tree.c | 32 ++++++++++++++++++++++++++++++++
2 files changed, 33 insertions(+), 42 deletions(-)
---
base-commit: e2e54005e20fb42d4a5e140d70a65be4a2363045
change-id: 20260912-fix-maple-tree-range64-rcu-02fa3d2e59a7
Best regards,
--
Dimitris Charisis <dchar@xxxxxxxxxxxxxxxxx>