CVE-2026-52996: ksmbd: fix durable fd leak on ClientGUID mismatch in durable v2 open

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

ksmbd: fix durable fd leak on ClientGUID mismatch in durable v2 open

ksmbdlookupfdcguid() returns a ksmbdfile with its refcount incremented via ksmbdfpget(). parsedurablehandlecontext() in the DURABLEREQV2 case properly releases this reference on every path inside the ClientGUID-match branch, either by calling ksmbdputdurablefd() or by transferring ownership to dhinfo->fp for a successful reconnect. However, when an entry exists in the global file table with the same CreateGuid but a different ClientGUID, the code simply falls through to the new-open path without dropping the reference obtained from ksmbdlookupfdcguid().

Per MS-SMB2 section 3.3.5.9.10 ("Handling the SMB2CREATEDURABLEHANDLEREQUESTV2 Create Context"), the server MUST locate an Open whose Open.CreateGuid matches the request's CreateGuid AND whose Open.ClientGuid matches the ClientGuid of the connection that received the request. If no such Open is found, the server MUST continue with the normal open execution phase. A CreateGuid hit with a ClientGUID mismatch is therefore the "Open not found" case: proceeding with a new open is correct, but the reference obtained purely as a side effect of the lookup must not be leaked.

Repeated requests that hit this mismatch pin globalft entries, prevent ksmbdclosefd() from ever running for the corresponding files, and defeat the durable scavenger, leading to long-lived resource leaks.

Release the reference in the mismatch path and clear dhinfo->fp so subsequent logic does not mistake a non-matching lookup result for a reconnect target.