CVE-2026-72085

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

scsi: xen: scsiback: Free unsubmitted command instead of double-putting it

scsibackgetpendreq() obtains a command tag and returns a vscsibkpend

whose embedded secmd has only been memset to 0, so its cmdkref is 0;

the secmd is initialised (krefinit() via targetinitcmd()) only

later, in scsibackcmdexec(), on the successful VSCSIIFACTSCSI_CDB

path. The two error paths in scsibackdocmd_fn() taken before the

command is submitted -- a failed scsibackgnttabdata_map() and an

unknown ring_req.act -- call

transportgenericfreecmd(&pendingreq->secmd, 0), which krefput()s a

refcount of 0. That underflows it ("refcount_t: underflow;

use-after-free") and, as the release function is not run, leaks the

command tag.

Impact: a pvSCSI guest can leak every command tag of a LUN's session,

stopping the LUN, by submitting requests with a bad grant reference or

an unknown request type; under paniconwarn the refcount underflow

panics the host.

Add a helper that just returns the tag with targetfreetag() and sends

the error response. It frees the tag while the v2p reference still pins

the session, and snapshots the response fields beforehand because

freeing the tag can let another ring reuse the pending_req slot.