CVE-2026-72085

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

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

scsiback_get_pend_req() obtains a command tag and returns a vscsibk_pend

whose embedded se_cmd has only been memset to 0, so its cmd_kref is 0;

the se_cmd is initialised (kref_init() via target_init_cmd()) only

later, in scsiback_cmd_exec(), on the successful VSCSIIF_ACT_SCSI_CDB

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

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

unknown ring_req.act -- call

transport_generic_free_cmd(&pending_req->se_cmd, 0), which kref_put()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 panic_on_warn the refcount underflow

panics the host.

Add a helper that just returns the tag with target_free_tag() 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.