on success or an NFSv4 status code on failure.
+ */
+__be32 nfsd_map_name_to_uid(struct svc_rqst *rqstp, const char *name,
+			    size_t namelen, kuid_t *uid)
 {
 	__be32 status;
 	u32 id = -1;
 
+	/*
+	 * The idmap lookup below triggers an upcall that invokes
+	 * cache_check(). RQ_USEDEFERRAL must be clear to prevent
+	 * cache_check() from setting RQ_DROPME via svc_defer().
+	 * NFSv4 servers are not permitted to drop requests. Also
+	 * RQ_DROPME will force NFSv4.1 session slot processing to
+	 * be skipped.
+	 */
+	WARN_ON_ONCE(test_bit(RQ_USEDEFERRAL, &rqstp->rq_flags));
+
 	if (name == NULL || namelen == 0)
 		return nfserr_inval;
 
@@ -660,13 +678,31 @@ nfsd_map_name_to_uid(struct svc_rqst *rqstp, const char *name, size_t namelen,
 	return status;
 }
 
-__be32
-nfsd_map_name_to_gid(struct svc_rqst *rqstp, const char *name, size_t namelen,
-		kgid_t *gid)
+/**
+ * nfsd_map_name_to_gid - Map user@domain to local GID
+ * @rqstp: RPC execution context
+ * @name: user@domain name to be mapped
+ * @namelen: length of name, in bytes
+ * @gid: OUT: mapped local GID value
+ *
+ * Returns nfs_ok on success or an NFSv4 status code on failure.
+ */
+__be32 nfsd_map_name_to_gid(struct svc_rqst *rqstp, const char *name,
+			    size_t namelen, kgid_t *gid)
 {
 	__be32 status;
 	u32 id = -1;
 
+	/*
+	 * The idmap lookup below triggers an upcall that invokes
+	 * cache_check(). RQ_USEDEFERRAL must be clear to prevent
+	 * cache_check() from setting RQ_DROPME via svc_defer().
+	 * NFSv4 servers are not permitted to drop requests. Also
+	 * RQ_DROPME will force NFSv4.1 session slot processing to
+	 * be skipped.
+	 */
+	WARN_ON_ONCE(test_bit(RQ_USEDEFERRAL, &rqstp->rq_flags));
+
 	if (name == NULL || namelen == 0)
 		return nfserr_inval;
 
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 42a6b914c0fe6..8dada7ef97cb1 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -2995,8 +2995,6 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 	BUG_ON(cstate->replay_owner);
 out:
 	cstate->status = status;
-	/* Reset deferral mechanism for RPC deferrals */
-	set_bit(RQ_USEDEFERRAL, &rqstp->rq_flags);
 	return rpc_success;
 }
 
diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index 4a403ce4fd468..5f046d5be4a6e 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -6001,6 +6001,22 @@ nfs4svc_decode_compoundargs(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 	args->ops = args->iops;
 	args->rqstp = rqstp;
 
+	/*
+	 * NFSv4 operation decoders can invoke svc cache lookups
+	 * that trigger svc_defer() when RQ_USEDEFERRAL is set,
+	 * setting RQ_DROPME. This creates two problems:
+	 *
+	 * 1. Non-idempotency: Compounds make it too hard to avoid
+	 *    problems if a request is deferred and replayed.
+	 *
+	 * 2. Session slot leakage (NFSv4.1+): If RQ_DROPME is set
+	 *    during decode but SEQUENCE executes successfully, the
+	 *    session slot will be marked INUSE. The request is then
+	 *    dropped before encoding, so the slot is never released,
+	 *    rendering it permanently unusable by the client.
+	 */
+	clear_bit(RQ_USEDEFERRAL, &rqstp->rq_flags);
+
 	return nfsd4_decode_compound(args);
 }
 
-- 
2.51.0[PATCH 6.18 274/641] nfsd: never defer requests during idmap lookupGreg Kroah-Hartman <gregkh@linuxfoundation.org> undefinedstable@vger.kernel.org undefined undefined undefined undefined undefined undefinedĽ