[PATCH] cifs: fix DFS traversal oops without CONFIG_CIFS_DFS_UPCALL

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When compiled with CONFIG_CIFS_DFS_UPCALL disabled, cifs_dfs_d_automount
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@xxxxxxx>
---
 fs/cifs/cifsfs.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/cifs/cifsfs.h b/fs/cifs/cifsfs.h
index 71fe0a0a7992..415176b2cf32 100644
--- a/fs/cifs/cifsfs.h
+++ b/fs/cifs/cifsfs.h
@@ -124,7 +124,10 @@ extern const struct dentry_operations cifs_ci_dentry_ops;
 #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 */
-- 
2.35.3




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