CVE-2021-47506

In the Linux kernel, the following vulnerability has been resolved: nfsd:
fix use-after-free due to delegation race A delegation break could arrive
as soon as we’ve called vfs_setlease. A delegation break runs a callback
which immediately (in nfsd4_cb_recall_prepare) adds the delegation to
del_recall_lru. If we then exit nfs4_set_delegation without hashing the
delegation, it will be freed as soon as the callback is done with it,
without ever being removed from del_recall_lru. Symptoms show up later as
use-after-free or list corruption warnings, usually in the laundromat
thread. I suspect aba2072f4523 “nfsd: grant read delegations to clients
holding writes” made this bug easier to hit, but I looked as far back as
v3.0 and it looks to me it already had the same problem. So I’m not sure
where the bug was introduced; it may have been there from the beginning.