[PATCH 4.14 56/66] cifs: fix DFS traversal oops without CONFIG_CIFS_DFS_UPCALL

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

From: David Disseldorp <ddiss@suse.de>

commit 179a88a8558bbf42991d361595281f3e45d7edfc upstream.

When compiled with CONFIG_CIFS_DFS_UPCALL disabled, cifs_dfs_d_automount
is NULL. cifs.ko logic for mapping CIFS_FATTR_DFS_REFERRAL attributes to
S_AUTOMOUNT and corresponding dentry flags is retained regardless of
CONFIG_CIFS_DFS_UPCALL, leading to a NULL pointer dereference in
VFS follow_automount() when traversing a DFS referral link:
  BUG: kernel NULL pointer dereference, address: 0000000000000000
  ...
  Call Trace:
   <TASK>
   __traverse_mounts+0xb5/0x220
   ? cifs_revalidate_mapping+0x65/0xc0 [cifs]
   step_into+0x195/0x610
   ? lookup_fast+0xe2/0xf0
   path_lookupat+0x64/0x140
   filename_lookup+0xc2/0x140
   ? __create_object+0x299/0x380
   ? kmem_cache_alloc+0x119/0x220
   ? user_path_at_empty+0x31/0x50
   user_path_at_empty+0x31/0x50
   __x64_sys_chdir+0x2a/0xd0
   ? exit_to_user_mode_prepare+0xca/0x100
   do_syscall_64+0x42/0x90
   entry_SYSCALL_64_after_hwframe+0x72/0xdc

This fix adds an inline cifs_dfs_d_automount() {return -EREMOTE} handler
when CONFIG_CIFS_DFS_UPCALL is disabled. An alternative would be to
avoid flagging S_AUTOMOUNT, etc. without CONFIG_CIFS_DFS_UPCALL. This
approach was chosen as it provides more control over the error path.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Cc: stable@vger.kernel.org
Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/cifs/cifsfs.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/fs/cifs/cifsfs.h
+++ b/fs/cifs/cifsfs.h
@@ -122,7 +122,10 @@ extern const struct dentry_operations ci
 #ifdef CONFIG_CIFS_DFS_UPCALL
 extern struct vfsmount *cifs_dfs_d_automount(struct path *path);
 #else
-#define cifs_dfs_d_automount NULL
+static inline struct vfsmount *cifs_dfs_d_automount(struct path *path)
+{
+	return ERR_PTR(-EREMOTE);
+}
 #endif
 
 /* Functions related to symlinks */