CVE-2026-90171 smb: smbdirect: release pending child sockets outside the handler lock
A use-after-free in the Linux kernel's SMB Direct module can cause a system crash during listener shutdown, requiring high privileges to trigger.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-90171?
CVE-2026-90171 is an use after free vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
smb: smbdirect: release pending child sockets outside the handler lock
smbdirect_socket_destroy() releases the listener's pending/ready child sockets while still holding the listener's handler lock, the &id_priv->handler_mutex taken via rdma_lock_handler(), not sc->listen.lock, and before the listener's own rdma_destroy_id(). That ordering has one real consequence and one cosmetic one.
The real one: smbdirect_socket_release() drops the child's last reference, which destroys the child's cm_id. Doing that before the listener's rdma_destroy_id() lets _cma_cancel_listens(), running from the listener's _destroy_id(), walk an already freed child id_priv, which KASAN catches as a slab-use-after-free during listener shutdown:
[ 4758.909130] BUG: KASAN: slab-use-after-free in __mutex_lock+0x1469/0x1560 [ 4758.911450] Read of size 1 at addr ffff88821c381db4 by task ksmbd.control/1652 [ 4758.913262] Call Trace: [ 4758.913267] <TASK> [ 4758.913299] __mutex_lock+0x1469/0x1560 [ 4758.913408] _cma_cancel_listens+0x312/0x3b0 [ 4758.913413] _destroy_id+0x363/0xee0 [ 4758.913417] smbdirect_socket_destroy_sync+0x17d5/0x2440 [ 4758.913443] smbdirect_socket_release+0x124/0x230 [ 4758.913451] ksmbd_rdma_stop_listening+0x9f/0x190 [ 4758.913457] ksmbd_conn_transport_destroy+0x65/0x3c0 [ 4758.913463] kill_server_store+0x1fb/0x2b0 [ 4758.913501] kernfs_fop_write_iter+0x349/0x4d0 [ 4758.913507] vfs_write+0x5e7/0xc70 [ 4758.913528] ksys_write+0x12a/0x210 [ 4758.913541] do_syscall_64+0x135/0x460 [ 4758.913555] entry_SYSCALL_64_after_hwframe+0x77/0x7f
The cosmetic one: releasing a child recurses into smbdirect_socket_destroy(), which takes the child's own rdma_lock_handler() lock nested under the listener's. The listener's and the child's cm_id are always different instances, so this cannot deadlock for real; the CM core itself nests a new connection id's handler_mutex under the listening id's in cma_ib_req_handler(). But lockdep only sees one lock class, reports possible recursive locking, and then disables itself, hiding real locking bugs for the rest of the run:
[ 2424.579653] WARNING: possible recursive locking detected [ 2424.581180] 7.1.0-next-20260623+ #89 Not tainted [ 2424.582548] -------------------------------------------- [ 2424.584500] ksmbd.control/8854 is trying to acquire lock: [ 2424.586817] ffff888102303c20 (&id_priv->handler_mutex){+.+.}-{4:4}, at: smbdirect_socket_destroy_sync+0xc39/0x2440 [ 2424.590590] [ 2424.590590] but task is already holding lock: [ 2424.591601] ffff888102046c20 (&id_priv->handler_mutex){+.+.}-{4:4}, at: smbdirect_socket_destroy_sync+0xc39/0x2440 [ 2424.594178] [ 2424.594178] other info that might help us debug this: [ 2424.596634] Possible unsafe locking scenario: [ 2424.596634] [ 2424.598841] CPU0 [ 2424.599765] ---- [ 2424.600695] lock(&id_priv->handler_mutex); [ 2424.601836] lock(&id_priv->handler_mutex); [ 2424.602590] [ 2424.602590] *** DEADLOCK *** [ 2424.602590] [ 2424.604512] May be due to missing lock nesting notation
Splice the pending/ready children onto a local list under the listener's listen.lock, while the handler lock is held so a concurrent CM CONNECT_REQUEST cannot add more, but defer the actual smbdirect_socket_release() calls until after the listener's cm_id has been destroyed and its handler lock dropped. The children are independent sockets whose teardown needs neither the listener's handler lock nor its cm_id.
Found with ksmbdzzer [2], a KSMBD fuzzer that drives libFuzzer with a kcov-dataflow [1] coverage vector: it folds each instrumented comparison/argument's runtime operand value together with its PC (the default arm mixes them as pc⊕val) so that a new operand value at a known site counts as new coverage.
[1] https://lwn.net/Articles/1077606/ [2] https://github.com/yskzalloc/kcov-dataflow
Which versions are affected by CVE-2026-90171?
How do I fix CVE-2026-90171?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-90171 being exploited?
There is no known exploitation of CVE-2026-90171 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-90171, GHSA-f5r6-6w97-7r58, UBUNTU-CVE-2026-90171.