Patch "NFS: don't try to cross a mountpount when there isn't one there." has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

NFS: don't try to cross a mountpount when there isn't one there.

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
nfs-don-t-try-to-cross-a-mountpount-when-there-isn-t-one-there.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: NeilBrown <[email protected]>
Date: Wed, 15 Mar 2017 12:40:44 +1100
Subject: NFS: don't try to cross a mountpount when there isn't one there.

From: NeilBrown <[email protected]>

[ Upstream commit 99bbf6ecc694dfe0b026e15359c5aa2a60b97a93 ]

consider the sequence of commands:
mkdir -p /import/nfs /import/bind /import/etc
mount --bind / /import/bind
mount --make-private /import/bind
mount --bind /import/etc /import/bind/etc

exportfs -o rw,no_root_squash,crossmnt,async,no_subtree_check localhost:/ mount -o vers=4 localhost:/ /import/nfs
ls -l /import/nfs/etc

You would not expect this to report a stale file handle. Yet it does.

The manipulations under /import/bind cause the dentry for /etc to get the DCACHE_MOUNTED flag set, even though nothing is mounted on /etc. This causes nfsd to call nfsd_cross_mnt() even though there is no mountpoint. So an upcall to mountd for "/etc" is performed.

The 'crossmnt' flag on the export of / causes mountd to report that /etc is exported as it is a descendant of /. It assumes the kernel wouldn't ask about something that wasn't a mountpoint. The filehandle returned identifies the filesystem and the inode number of /etc.

When this filehandle is presented to rpc.mountd, via "nfsd.fh", the inode cannot be found associated with any name in /etc/exports, or with any mountpoint listed by getmntent(). So rpc.mountd says the filehandle doesn't exist. Hence ESTALE.

This is fixed by teaching nfsd not to trust DCACHE_MOUNTED too much. It is just a hint, not a guarantee.
Change nfsd_mountpoint() to return '1' for a certain mountpoint, '2' for a possible mountpoint, and 0 otherwise.

Then change nfsd_crossmnt() to check if follow_down() actually found a mountpount and, if not, to avoid performing a lookup if the location is not known to certainly require an export-point.

---
 fs/nfsd/vfs.c |   24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

@@ -94,6 +94,12 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, s
 	err = follow_down(&path);
 	if (err < 0)
 		goto out;
+	if (path.mnt == exp->ex_path.mnt && path.dentry == dentry &&
+	    nfsd_mountpoint(dentry, exp) == 2) {
+		/* This is only a mountpoint in some other namespace */
+		path_put(&path);
+		goto out;
+	}
 
 	exp2 = rqst_exp_get_by_name(rqstp, &path);
 	if (IS_ERR(exp2)) {

@@ -167,16 +173,26 @@ static int nfsd_lookup_parent(struct svc
 /*
  * For nfsd purposes, we treat V4ROOT exports as though there was an
  * export at *every* directory.
+ * We return:
+ * '1' if this dentry *must* be an export point,
+ * '2' if it might be, if there is really a mount here, and
+ * '0' if there is no chance of an export point here.
  */
 int nfsd_mountpoint(struct dentry *dentry, struct svc_export *exp)
 {
-	if (d_mountpoint(dentry))
+	if (!d_inode(dentry))
+		return 0;
+	if (exp->ex_flags & NFSEXP_V4ROOT)
 		return 1;
 	if (nfsd4_is_junction(dentry))
 		return 1;
-	if (!(exp->ex_flags & NFSEXP_V4ROOT))
-		return 0;
-	return d_inode(dentry) != NULL;
+	if (d_mountpoint(dentry))
+		/*
+		 * Might only be a mountpoint in a different namespace,
+		 * but we need to check.
+		 */
+		return 2;
+	return 0;
 }
 
 __be32

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/dm-ensure-bio-submission-follows-a-depth-first-tree-walk.patch queue-4.9/md-raid10-wait-up-frozen-array-in-handle_write_completed.patch queue-4.9/nfs-don-t-try-to-cross-a-mountpount-when-there-isn-t-one-there.patch queue-4.9/md-raid10-skip-spare-disk-as-first-disk.patch