xen/scsifront: don't use gnttab_query_foreign_access() for mapped status

It isn't enough to check whether a grant is still being in use by
calling gnttab_query_foreign_access(), as a mapping could be realized
by the other side just after having called that function.

In case the call was done in preparation of revoking a grant it is
better to do so via gnttab_try_end_foreign_access() and check the
success of that operation instead.

This is CVE-2022-23038 / part of XSA-396.
Reported-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
V2:
- use gnttab_try_end_foreign_access()

drivers/scsi/xen-scsifront.c

@@ -233,12 +233,11 @@ static void scsifront_gnttab_done(struct vscsifrnt_info *info,
 		return;
 
 	for (i = 0; i < shadow->nr_grants; i++) {
-		if (unlikely(gnttab_query_foreign_access(shadow->gref[i]))) {
+		if (unlikely(!gnttab_try_end_foreign_access(shadow->gref[i]))) {
 			shost_printk(KERN_ALERT, info->host, KBUILD_MODNAME
 				     "grant still in use by backend\n");
 			BUG();
 		}
-		gnttab_end_foreign_access(shadow->gref[i], 0, 0UL);
 	}
 
 	kfree(shadow->sg);