ksmbd: fix parent dentry lifetime in directory enumeration

ksmbd_populate_dot_dotdot_entries() builds the ".." entry from the open directory's d_parent and passes that pointer to ksmbd_vfs_fill_dentry_attrs() without taking a reference. That pointer is only valid while the directory stays under this parent. If a concurrent rename moves the directory elsewhere, the old parent loses its last reference; a following rmdir then turns it into a negative dentry with d_inode cleared, and after the RCU grace period the dentry itself is freed. The worker still holds the raw pointer and dereferences d_inode through vfs_getattr() and vfs_getxattr().

A remote SMB client, authenticated or guest, triggers this by listing a directory while that directory is moved under a new parent and the old parent is deleted. It can be done by the client itself with write access or by other concurrent writers, triggering null-ptr-deref and slab-use-after-free on a KASAN build.

  KASAN: null-ptr-deref in range
      [0x0000000000000000-0x0000000000000007]
  Workqueue: ksmbd-io handle_ksmbd_work
  RIP: 0010:xattr_permission+0x9d/0x160
  Call Trace:
   vfs_getxattr+0x9c/0x1b0
   ksmbd_vfs_getxattr+0x82/0x100
   ksmbd_vfs_get_dos_attrib_xattr+0x7f/0x130
   ksmbd_vfs_fill_dentry_attrs+0x435/0x650
   ksmbd_populate_dot_dotdot_entries+0x25e/0x630
   smb2_query_dir+0xcf9/0x1a20
   handle_ksmbd_work+0x5f1/0x1c80
   process_one_work+0x90b/0x19d0

Fix by using the dentry reference counting APIs: take the parent with dget_parent(), take a matching dget() on ".", and dput() the dentry once ksmbd_vfs_fill_dentry_attrs() has returned or the pattern does not match.

---
 fs/smb/server/smb_common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index ec6ee56293ce..8e44b15a4177 100644
@@ -478,16 +478,17 @@ int ksmbd_populate_dot_dotdot_entries(struct ksmbd_work *work, int info_level,
 			if (i == 0) {
 				d_info->name = ".";
 				d_info->name_len = 1;
-				dentry = dir->filp->f_path.dentry;
+				dentry = dget(dir->filp->f_path.dentry);
 			} else {
 				d_info->name = "..";
 				d_info->name_len = 2;
-				dentry = dir->filp->f_path.dentry->d_parent;
+				dentry = dget_parent(dir->filp->f_path.dentry);
 			}
 
 			if (!match_pattern(d_info->name, d_info->name_len,
 					   search_pattern)) {
 				dir->dot_dotdot[i] = 1;
+				dput(dentry);
 				continue;
 			}
 

@@ -496,6 +497,7 @@ int ksmbd_populate_dot_dotdot_entries(struct ksmbd_work *work, int info_level,
 							 idmap,
 							 dentry,
 							 &ksmbd_kstat);
+			dput(dentry);
 			if (rc)
 				break;
 

2.55.0

Applied it to #ksmbd-for-next.
Thanks!