CVE-2026-46107

In the Linux kernel, the following vulnerability has been resolved:

dm-thin: fix metadata refcount underflow

There's a bug in dm-thin in the function rebalance_children. If the

internal btree node has one entry, the code tries to copy all btree

entries from the node's child to the node itself and then decrement the

child's reference count.

If the child node is shared (it has reference count > 1), we won't free

it, so there would be two pointers to each of the grandchildren nodes.

But the reference counts of the grandchildren is not increased, thus the

reference count doesn't match the number of pointers that point to the

grandchildren. This results in "device mapper: space map common: unable

to decrement block" errors.

Fix this bug by incrementing reference counts on the grandchildren if the

btree node is shared.