CVE-2026-64142

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

ksmbd: close durable scavenger races against m_fp_list lookups

ksmbd_durable_scavenger() has two related races against any walker

that iterates f_ci->m_fp_list, including ksmbd_lookup_fd_inode()

(used by ksmbd_vfs_rename) and the share-mode checks in

fs/smb/server/smb_common.c.

(1) fp->node list-head reuse. Durable-preserved handles can remain

linked on f_ci->m_fp_list after session teardown so share-mode checks

still see them while the handle is reconnectable. The scavenger

collected expired handles by adding fp->node to a local

scavenger_list after removing them from the global durable idr.

Because fp->node is the same list_head used by m_fp_list,

list_add(&fp->node, &scavenger_list) overwrites the m_fp_list links

and corrupts both lists. CONFIG_DEBUG_LIST can report this on the

share-mode walk path.

(2) Refcount race against m_fp_list walkers. The scavenger qualifies

an expired durable handle with atomic_read(&fp->refcount) > 1 and

fp->conn under global_ft.lock, removes fp from global_ft, then drops

global_ft.lock before unlinking fp from m_fp_list and freeing it.

During that gap fp is still linked on m_fp_list with f_state ==

FP_INITED. ksmbd_lookup_fd_inode() under m_lock read calls

ksmbd_fp_get() (atomic_inc_not_zero on refcount that is still 1) and

takes a live reference; the scavenger then unlinks and frees fp

while the holder owns a reference, leading to UAF on the holder's

subsequent ksmbd_fd_put() and on any field reads performed by a

concurrent share-mode walker that iterates m_fp_list without taking

ksmbd_fp_get() (smb_check_perm_dleases-like paths).

Fix both:

* Stop reusing fp->node as a scavenger-private list node. Remove

one expired handle from global_ft under global_ft.lock, take an

explicit transient reference, drop the lock, unlink fp->node

from m_fp_list under f_ci->m_lock, then drop both the durable

lifetime and transient references with atomic_sub_and_test(2,

&fp->refcount). If the scavenger is the last putter the close

runs there; otherwise an in-flight holder that already raced

through the m_fp_list lookup owns the final close via its

ksmbd_fd_put() path. The one-at-a-time disposal can rescan the

durable idr when multiple handles expire in the same pass, but

durable scavenging is a background expiration path and the final

full scan recomputes min_timeout before the next wait.

* Clear fp->persistent_id inside __ksmbd_remove_durable_fd() right

after idr_remove(), so a delayed final close from a holder that

snatched fp does not re-issue idr_remove() on a persistent id

that idr_alloc_cyclic() in ksmbd_open_durable_fd() may have

already handed out to a brand-new durable handle.

* Bypass the per-conn open_files_count decrement in

__put_fd_final() when fp is detached from any session table

(fp->conn cleared by session_fd_check() at durable preserve --

paired with the volatile_id clear at unpublish, so checking

fp->conn alone is sufficient). The walker that owns the final

close runs from an unrelated work->conn whose

stats.open_files_count never tracked this durable fp; without

this guard the holder would underflow that unrelated counter.

The two races are folded into one patch because patch (1) alone

cleans up the corrupted list but leaves a deterministic UAF window

for m_fp_list walkers that the transient-reference and

persistent_id discipline in (2) close; bisecting onto an

intermediate state would land on a UAF that pre-patch chaos merely

made less reproducible.

Validation:

* CONFIG_DEBUG_LIST coverage for the list_head reuse path.

* KASAN-enabled direct SMB2 durable-handle coverage that exercised

ksmbd_durable_scavenger() and non-NULL ksmbd_lookup_fd_inode()

returns while durable handles expired under concurrent rename

lookups, with no KASAN, UAF, list-corruption, ODEBUG, or WARNING

reports.

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