NFSD: Force all NFSv4.2 COPY requests to be synchronous

JIRA: https://issues.redhat.com/browse/RHEL-64496
CVE: CVE-2024-49974
We're addressing this CVE by including an upstream fix that disables
async copy because the fix for async copy upstream is still incomplete.
Once async copy has been completely fixed upstream, we will include
the upstream patch that re-enables it in RHEL and include all the fixes.

commit 8d915bb
Author: Chuck Lever <chuck.lever@oracle.com>
Date:   Tue May 7 09:37:14 2024 -0400

    NFSD: Force all NFSv4.2 COPY requests to be synchronous

    We've discovered that delivering a CB_OFFLOAD operation can be
    unreliable in some pretty unremarkable situations. Examples
    include:

     - The server dropped the connection because it lost a forechannel
       NFSv4 request and wishes to force the client to retransmit
     - The GSS sequence number window under-flowed
     - A network partition occurred

    When that happens, all pending callback operations, including
    CB_OFFLOAD, are lost. NFSD does not retransmit them.

    Moreover, the Linux NFS client does not yet support sending an
    OFFLOAD_STATUS operation to probe whether an asynchronous COPY
    operation has finished. Thus, on Linux NFS clients, when a
    CB_OFFLOAD is lost, asynchronous COPY can hang until manually
    interrupted.

    I've tried a couple of remedies, but so far the side-effects are
    worse than the disease and they have had to be reverted. So
    temporarily force COPY operations to be synchronous so that the use
    of CB_OFFLOAD is avoided entirely. This is a fix that can easily be
    backported to LTS kernels. I am working on client patches that
    introduce an implementation of OFFLOAD_STATUS.

    Note that NFSD arbitrarily limits the size of a copy_file_range
    to 4MB to avoid indefinitely blocking an nfsd thread. A short
    COPY result is returned in that case, and the client can present
    a fresh COPY request for the remainder.

    Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>

Author: Olga Kornievskaia

fs/nfsd/nfs4proc.c

@@ -1828,6 +1828,13 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	__be32 status;
 	struct nfsd4_copy *async_copy = NULL;
 
+	/*
+	 * Currently, async COPY is not reliable. Force all COPY
+	 * requests to be synchronous to avoid client application
+	 * hangs waiting for COPY completion.
+	 */
+	nfsd4_copy_set_sync(copy, true);
+
 	copy->cp_clp = cstate->clp;
 	if (nfsd4_ssc_is_inter(copy)) {
 		trace_nfsd_copy_inter(copy);