[PATCH 5.10 01/79] NFSD: Fix verifier returned in stable WRITEs

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Chuck Lever <chuck.lever@oracle.com>

commit f11ad7aa653130b71e2e89bed207f387718216d5 upstream.

RFC 8881 explains the purpose of the write verifier this way:

> The final portion of the result is the field writeverf. This field
> is the write verifier and is a cookie that the client can use to
> determine whether a server has changed instance state (e.g., server
> restart) between a call to WRITE and a subsequent call to either
> WRITE or COMMIT.

But then it says:

> This cookie MUST be unchanged during a single instance of the
> NFSv4.1 server and MUST be unique between instances of the NFSv4.1
> server. If the cookie changes, then the client MUST assume that
> any data written with an UNSTABLE4 value for committed and an old
> writeverf in the reply has been lost and will need to be
> recovered.

RFC 1813 has similar language for NFSv3. NFSv2 does not have a write
verifier since it doesn't implement the COMMIT procedure.

Since commit 19e0663ff9bc ("nfsd: Ensure sampling of the write
verifier is atomic with the write"), the Linux NFS server has
returned a boot-time-based verifier for UNSTABLE WRITEs, but a zero
verifier for FILE_SYNC and DATA_SYNC WRITEs. FILE_SYNC and DATA_SYNC
WRITEs are not followed up with a COMMIT, so there's no need for
clients to compare verifiers for stable writes.

However, by returning a different verifier for stable and unstable
writes, the above commit puts the Linux NFS server a step farther
out of compliance with the first MUST above. At least one NFS client
(FreeBSD) noticed the difference, making this a potential
regression.

[Removed down_write to fix the conflict in the cherry-pick. The
down_write functionality was no longer needed there. Upstream commit
555dbf1a9aac6d3150c8b52fa35f768a692f4eeb titled nfsd: Replace use of
rwsem with errseq_t removed those and replace it with new functionality
that was more scalable. This commit is already backported onto 5.10 and
so removing down_write ensures consistency with that change. Tested by
compiling and booting successfully. - kochera]

Reported-by: Rick Macklem <rmacklem@uoguelph.ca>
Link: https://lore.kernel.org/linux-nfs/YQXPR0101MB096857EEACF04A6DF1FC6D9BDD749@YQXPR0101MB0968.CANPRD01.PROD.OUTLOOK.COM/T/
Fixes: 19e0663ff9bc ("nfsd: Ensure sampling of the write verifier is atomic with the write")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Michael Kochera <kochera@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/nfsd/vfs.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1014,6 +1014,10 @@ nfsd_vfs_write(struct svc_rqst *rqstp, s
 	iov_iter_kvec(&iter, WRITE, vec, vlen, *cnt);
 	since = READ_ONCE(file->f_wb_err);
 	if (flags & RWF_SYNC) {
+		if (verf)
+			nfsd_copy_boot_verifier(verf,
+					net_generic(SVC_NET(rqstp),
+					nfsd_net_id));
 		host_err = vfs_iter_write(file, &iter, &pos, flags);
 		if (host_err < 0)
 			nfsd_reset_boot_verifier(net_generic(SVC_NET(rqstp),