CVE-2026-64142: ksmbd: close durable scavenger races against m_fp_list lookups

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

ksmbd: close durable scavenger races against mfplist lookups

ksmbddurablescavenger() has two related races against any walker that iterates fci->mfplist, including ksmbdlookupfdinode() (used by ksmbdvfsrename) and the share-mode checks in fs/smb/server/smbcommon.c.

(1) fp->node list-head reuse. Durable-preserved handles can remain linked on fci->mfplist 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 scavengerlist after removing them from the global durable idr. Because fp->node is the same listhead used by mfplist, listadd(&fp->node, &scavengerlist) overwrites the mfplist links and corrupts both lists. CONFIGDEBUGLIST can report this on the share-mode walk path.

(2) Refcount race against mfplist walkers. The scavenger qualifies an expired durable handle with atomicread(&fp->refcount) > 1 and fp->conn under globalft.lock, removes fp from globalft, then drops globalft.lock before unlinking fp from mfplist and freeing it. During that gap fp is still linked on mfplist with fstate == FPINITED. ksmbdlookupfdinode() under mlock read calls ksmbdfpget() (atomicincnotzero 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 ksmbdfdput() and on any field reads performed by a concurrent share-mode walker that iterates mfplist without taking ksmbdfpget() (smbcheckpermdleases-like paths).

Fix both:

Stop reusing fp->node as a scavenger-private list node. Remove one expired handle from globalft under globalft.lock, take an explicit transient reference, drop the lock, unlink fp->node from mfplist under fci->mlock, then drop both the durable lifetime and transient references with atomicsubandtest(2, &fp->refcount). If the scavenger is the last putter the close runs there; otherwise an in-flight holder that already raced through the mfplist lookup owns the final close via its ksmbdfdput() 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 mintimeout before the next wait.

Clear fp->persistentid inside ksmbdremovedurablefd() right after idrremove(), so a delayed final close from a holder that snatched fp does not re-issue idrremove() on a persistent id that idralloccyclic() in ksmbdopendurablefd() may have already handed out to a brand-new durable handle.

Bypass the per-conn openfilescount decrement in putfdfinal() when fp is detached from any session table (fp->conn cleared by sessionfdcheck() at durable preserve -- paired with the volatileid 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.openfilescount 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 mfplist walkers that the transient-reference and persistentid discipline in (2) close; bisecting onto an intermediate state would land on a UAF that pre-patch chaos merely made less reproducible.

Validation: CONFIGDEBUGLIST coverage for the listhead reuse path. KASAN-enabled direct SMB2 durable-handle coverage that exercised ksmbddurablescavenger() and non-NULL ksmbdlookupfdinode() returns while durable handles expired under concurrent rename lookups, with no KASAN, UAF, list-corruption, ODEBUG, or WARNING reports. ---truncated---