CVE-2026-74513: dibs: fix use-after-free of dmb_node in loopback attach/detach/unregister

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

dibs: fix use-after-free of dmb_node in loopback attach/detach/unregister

dibs_lo_attach_dmb(), dibs_lo_detach_dmb() and dibs_lo_unregister_dmb()
look up the dmb_node under dmb_ht_lock, drop the lock and only then
operate on the node's refcount. Nothing keeps the node alive across
that window: __dibs_lo_unregister_dmb() removes the node from the hash
table under the write lock and immediately frees it.

A concurrent final put can therefore free the node between the lookup
and the refcount operation:

CPU0 (attach)                     CPU1 (owner unregisters)

read_lock_bh(&dmb_ht_lock)
find dmb_node (refcnt == 1)
read_unlock_bh(&dmb_ht_lock)
                                  refcount_dec_and_test() 1 -> 0
                                  write_lock_bh(&dmb_ht_lock)
                                  hash_del(&dmb_node->list)
                                  write_unlock_bh(&dmb_ht_lock)
                                  kfree(dmb_node)
refcount_inc_not_zero(&dmb_node->refcnt)  <-- use-after-free

The same window exists for the refcount_dec_and_test() calls in the
detach and unregister paths.

Close the race structurally by making hash table membership and the
refcount transitions atomic with respect to each other:

- Perform the final refcount_dec_and_test() and hash_del() in a single
  dmb_ht_lock write-side critical section, in both the unregister and
  the detach path. Freeing the node still happens after the lock is
  dropped, which is safe because a node whose refcount reached zero has
  left the hash table and can no longer be found.

- This establishes the invariant that any node found in the hash table
  holds at least one reference, and that the final reference can only
  be dropped under the write lock. dibs_lo_attach_dmb() can thus take
  its reference with a plain refcount_inc() while still holding the
  read lock; refcount_inc_not_zero() is no longer needed.

__dibs_lo_unregister_dmb() no longer touches the hash table and is
renamed to dibs_lo_free_dmb() accordingly.

Note: commit cc21191b584c ("dibs: Move data path to dibs layer") moved
the code to its current location; the race was introduced earlier by
commit c3a910f2380f ("net/smc: implement DMB-merged operations of
loopback-ism").

Tested SMC-D via ISM and dibs loopback.

The Linux kernel CVE team has assigned CVE-2026-74513 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.10 with commit c3a910f2380fe294d14e42af66af3d3eed8fecbf and fixed in 6.18.44 with commit c0837aeace96152d14b17fdd19d70102b6631a7d
	Issue introduced in 6.10 with commit c3a910f2380fe294d14e42af66af3d3eed8fecbf and fixed in 7.1.8 with commit 48c073f88c93707089a4214f21cb4c3de5aea6e4
	Issue introduced in 6.10 with commit c3a910f2380fe294d14e42af66af3d3eed8fecbf and fixed in 7.2-rc6 with commit a10ea943356b9d70c5616a0a06f6fa97cfdaccb1

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74513
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/dibs/dibs_loopback.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/c0837aeace96152d14b17fdd19d70102b6631a7d
	https://git.kernel.org/stable/c/48c073f88c93707089a4214f21cb4c3de5aea6e4
	https://git.kernel.org/stable/c/a10ea943356b9d70c5616a0a06f6fa97cfdaccb1