smb/server: avoid deadlock when linking with ReplaceIfExists

If smb2_create_link() is called with ReplaceIfExists set and the name
does exist then a deadlock will happen.

ksmbd_vfs_kern_path_locked() will return with success and the parent
directory will be locked.  ksmbd_vfs_remove_file() will then remove the
file.  ksmbd_vfs_link() will then be called while the parent is still
locked.  It will try to lock the same parent and will deadlock.

This patch moves the ksmbd_vfs_kern_path_unlock() call to *before*
ksmbd_vfs_link() and then simplifies the code, removing the file_present
flag variable.

Signed-off-by: NeilBrown <neil@brown.name>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: neilbrown

fs/smb/server/smb2pdu.c

@@ -6037,7 +6037,6 @@ static int smb2_create_link(struct ksmbd_work *work,
 {
 	char *link_name = NULL, *target_name = NULL, *pathname = NULL;
 	struct path path, parent_path;
-	bool file_present = false;
 	int rc;
 
 	if (buf_len < (u64)sizeof(struct smb2_file_link_info) +
@@ -6070,33 +6069,26 @@ static int smb2_create_link(struct ksmbd_work *work,
 	if (rc) {
 		if (rc != -ENOENT)
 			goto out;
-	} else
-		file_present = true;
-
-	if (file_info->ReplaceIfExists) {
-		if (file_present) {
+	} else {
+		if (file_info->ReplaceIfExists) {
 			rc = ksmbd_vfs_remove_file(work, &path);
 			if (rc) {
 				rc = -EINVAL;
 				ksmbd_debug(SMB, "cannot delete %s\n",
 					    link_name);
 				goto out;
 			}
-		}
-	} else {
-		if (file_present) {
+		} else {
 			rc = -EEXIST;
 			ksmbd_debug(SMB, "link already exists\n");
 			goto out;
 		}
+		ksmbd_vfs_kern_path_unlock(&parent_path, &path);
 	}
-
 	rc = ksmbd_vfs_link(work, target_name, link_name);
 	if (rc)
 		rc = -EINVAL;
 out:
-	if (file_present)
-		ksmbd_vfs_kern_path_unlock(&parent_path, &path);
 
 	if (!IS_ERR(link_name))
 		kfree(link_name);