[PATCH 1/2] xen/scsiback: free unsubmitted command instead of double-putting it

[Michael Bommarito <michael.bommarito@xxxxxxxxx>]

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.

Fixes: 2dbcdf33dbf6 ("xen-scsiback: Convert to percpu_ida tag allocation")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@xxxxxxxxx>
---
Reproduced on a Xen dom0 (Linux 6.1.y) exporting a pvSCSI LUN to a guest.
A frontend that sends a single ring request with an unknown action type
drives scsiback_do_cmd_fn() into transport_generic_free_cmd() on the
never-initialised command and logs

  refcount_t: underflow; use-after-free
  WARNING: ... refcount_warn_saturate
   transport_generic_free_cmd+0x... [target_core_mod]
   scsiback_do_cmd_fn+0x... [xen_scsiback]
   scsiback_irq_fn+0x... [xen_scsiback]

from the vscsiif IRQ thread, and panics the dom0 under panic_on_warn.  The
failed grant-map path reaches the same free.  With this patch the same
request is answered with DID_ERROR and the tag is returned, with no
underflow.  These error paths are unchanged since 2dbcdf33dbf6, so mainline
is affected identically.

 drivers/xen/xen-scsiback.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c
index e33f95c91b096..f324732eba7f8 100644
--- a/drivers/xen/xen-scsiback.c
+++ b/drivers/xen/xen-scsiback.c
@@ -611,6 +611,25 @@ static void scsiback_disconnect(struct vscsibk_info *info)
        xenbus_unmap_ring_vfree(info->dev, info->ring.sring);
 }
 
+/*
+ * Send the error response for a request that did not reach the target core
+ * and return its tag.  Free the tag before the response drops the v2p
+ * reference that keeps the session alive, and snapshot what the response
+ * needs since returning the tag can let the slot be reused.
+ */
+static void scsiback_resp_and_free(struct vscsibk_pend *pending_req,
+                                  int32_t result)
+{
+       struct vscsibk_info *info = pending_req->info;
+       struct v2p_entry *v2p = pending_req->v2p;
+       struct se_session *se_sess = v2p->tpg->tpg_nexus->tvn_se_sess;
+       u16 rqid = pending_req->rqid;
+
+       target_free_tag(se_sess, &pending_req->se_cmd);
+       scsiback_send_response(info, NULL, result, 0, rqid);
+       kref_put(&v2p->kref, scsiback_free_translation_entry);
+}
+
 static void scsiback_device_action(struct vscsibk_pend *pending_req,
        enum tcm_tmreq_table act, int tag)
 {
@@ -792,9 +811,8 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info,
                case VSCSIIF_ACT_SCSI_CDB:
                        if (scsiback_gnttab_data_map(&ring_req, pending_req)) {
                                scsiback_fast_flush_area(pending_req);
-                               scsiback_do_resp_with_sense(NULL,
-                                               DID_ERROR << 16, 0, 
pending_req);
-                               
transport_generic_free_cmd(&pending_req->se_cmd, 0);
+                               scsiback_resp_and_free(pending_req,
+                                                      DID_ERROR << 16);
                        } else {
                                scsiback_cmd_exec(pending_req);
                        }
@@ -808,9 +826,7 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info,
                        break;
                default:
                        pr_err_ratelimited("invalid request\n");
-                       scsiback_do_resp_with_sense(NULL, DID_ERROR << 16, 0,
-                                                   pending_req);
-                       transport_generic_free_cmd(&pending_req->se_cmd, 0);
+                       scsiback_resp_and_free(pending_req, DID_ERROR << 16);
                        break;
                }
 
-- 
2.53.0