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

nfsd: never defer requests during idmap lookup

During v4 request compound arg decoding, some ops (e.g. SETATTR)
can trigger idmap lookup upcalls. When those upcall responses get
delayed beyond the allowed time limit, cache_check() will mark the
request for deferral and cause it to be dropped.

This prevents nfs4svc_encode_compoundres from being executed, and
thus the session slot flag NFSD4_SLOT_INUSE never gets cleared.
Subsequent client requests will fail with NFSERR_JUKEBOX, given
that the slot will be marked as in-use, making the SEQUENCE op
fail.

Fix this by making sure that the RQ_USEDEFERRAL flag is always
clear during nfs4svc_decode_compoundargs(), since no v4 request
should ever be deferred.