[PATCH v3 2/2] nfsd: do not allow exporting of special kernel filesystems

[Amir Goldstein <amir73il@gmail.com>]

pidfs and nsfs recently gained support for encode/decode of file handles
via name_to_handle_at(2)/opan_by_handle_at(2).

These special kernel filesystems have custom ->open() and ->permission()
export methods, which nfsd does not respect and it was never meant to be
used for exporting those filesystems by nfsd.

Therefore, do not allow nfsd to export filesystems with custom ->open()
or ->permission() methods.

Fixes: b3caba8f7a34a ("pidfs: implement file handle support")
Fixes: 5222470b2fbb3 ("nsfs: support file handles")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/nfsd/export.c         | 8 +++++---
 include/linux/exportfs.h | 9 +++++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index 2a1499f2ad196..09fe268fe2c76 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -427,7 +427,8 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
 	 *       either a device number (so FS_REQUIRES_DEV needed)
 	 *       or an FSID number (so NFSEXP_FSID or ->uuid is needed).
 	 * 2:  We must be able to find an inode from a filehandle.
-	 *       This means that s_export_op must be set.
+	 *       This means that s_export_op must be set and comply with
+	 *       the requirements for remote filesystem export.
 	 * 3: We must not currently be on an idmapped mount.
 	 */
 	if (!(inode->i_sb->s_type->fs_flags & FS_REQUIRES_DEV) &&
@@ -437,8 +438,9 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
 		return -EINVAL;
 	}
 
-	if (!exportfs_can_decode_fh(inode->i_sb->s_export_op)) {
-		dprintk("exp_export: export of invalid fs type.\n");
+	if (!exportfs_may_export(inode->i_sb->s_export_op)) {
+		dprintk("exp_export: export of invalid fs type (%s).\n",
+			inode->i_sb->s_type->name);
 		return -EINVAL;
 	}
 
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index fafd22ed4c648..bf3dee2ad5f97 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -340,6 +340,15 @@ static inline bool exportfs_can_decode_fh(const struct export_operations *nop)
 	return nop && nop->fh_to_dentry;
 }
 
+static inline bool exportfs_may_export(const struct export_operations *nop)
+{
+	/*
+	 * Do not allow nfs export for filesystems with custom ->open() and
+	 * ->permission() ops, which nfsd does not respect (e.g. pidfs, nsfs).
+	 */
+	return exportfs_can_decode_fh(nop) && !nop->open && !nop->permission;
+}
+
 static inline bool exportfs_can_encode_fh(const struct export_operations *nop,
 					  int fh_flags)
 {
-- 
2.52.0