In the Linux kernel, the following vulnerability has been resolved:
NFS: Return a delegation the client fails to record
When an NFS server grants a delegation in an OPEN reply,
nfs_inode_set_delegation() records it on the client. However, three
of its error flows return without sending DELEGRETURN.
A delegation can be relinquished only by DELEGRETURN (RFC 8881
Section 20.2.4), so dropping one silently leaves the server believing
the client still holds it. If the server happens to recall that
delegation, the client answers CB_RECALL with NFS4ERR_BADHANDLE
because it has no record of the stateid. The server revokes the
delegation and moves it onto its cl_revoked list, because the client
never sends the FREE_STATEID that would drain it. Every subsequent
SEQUENCE reply then carries SEQ4_STATUS_RECALLABLE_STATE_REVOKED,
and the client's state manager loops issuing TEST_STATEID across its
delegations without ever clearing the condition.
The window is easy to reach now that a server offers a write
delegation on any write OPEN: a delegation recalled for one opener
races a re-open that the server answers with a fresh write
delegation.
Instead of dropping it, hand the delegation back during these error
flows.