CIFS: Fix a possible memory corruption during reconnect

We can not unlock/lock cifs_tcp_ses_lock while walking through ses
and tcon lists because it can corrupt list iterator pointers and
a tcon structure can be released if we don't hold an extra reference.
Fix it by moving a reconnect process to a separate delayed work
and acquiring a reference to every tcon that needs to be reconnected.
Also do not send an echo request on newly established connections.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>

1 files changed, 51 insertions, 24 deletions

diff --git a/fs/cifs/smb2pdu.c b/fs/cifs/smb2pdu.c
index 730d57b0ffd4..4ba3f68a1766 100644
--- a/fs/cifs/smb2pdu.c
+++ b/fs/cifs/smb2pdu.c
@@ -1972,6 +1972,54 @@ smb2_echo_callback(struct mid_q_entry *mid)
 	add_credits(server, credits_received, CIFS_ECHO_OP);
 }
 
+void smb2_reconnect_server(struct work_struct *work)
+{
+	struct TCP_Server_Info *server = container_of(work,
+					struct TCP_Server_Info, reconnect.work);
+	struct cifs_ses *ses;
+	struct cifs_tcon *tcon, *tcon2;
+	struct list_head tmp_list;
+	int tcon_exist = false;
+
+	/* Prevent simultaneous reconnects that can corrupt tcon->rlist list */
+	mutex_lock(&server->reconnect_mutex);
+
+	INIT_LIST_HEAD(&tmp_list);
+	cifs_dbg(FYI, "Need negotiate, reconnecting tcons\n");
+
+	spin_lock(&cifs_tcp_ses_lock);
+	list_for_each_entry(ses, &server->smb_ses_list, smb_ses_list) {
+		list_for_each_entry(tcon, &ses->tcon_list, tcon_list) {
+			if (tcon->need_reconnect) {
+				tcon->tc_count++;
+				list_add_tail(&tcon->rlist, &tmp_list);
+				tcon_exist = true;
+			}
+		}
+	}
+	/*
+	 * Get the reference to server struct to be sure that the last call of
+	 * cifs_put_tcon() in the loop below won't release the server pointer.
+	 */
+	if (tcon_exist)
+		server->srv_count++;
+
+	spin_unlock(&cifs_tcp_ses_lock);
+
+	list_for_each_entry_safe(tcon, tcon2, &tmp_list, rlist) {
+		smb2_reconnect(SMB2_ECHO, tcon);
+		list_del_init(&tcon->rlist);
+		cifs_put_tcon(tcon);
+	}
+
+	cifs_dbg(FYI, "Reconnecting tcons finished\n");
+	mutex_unlock(&server->reconnect_mutex);
+
+	/* now we can safely release srv struct */
+	if (tcon_exist)
+		cifs_put_tcp_session(server, 1);
+}
+
 int
 SMB2_echo(struct TCP_Server_Info *server)
 {
@@ -1984,32 +2032,11 @@ SMB2_echo(struct TCP_Server_Info *server)
 	cifs_dbg(FYI, "In echo request\n");
 
 	if (server->tcpStatus == CifsNeedNegotiate) {
-		struct list_head *tmp, *tmp2;
-		struct cifs_ses *ses;
-		struct cifs_tcon *tcon;
-
-		cifs_dbg(FYI, "Need negotiate, reconnecting tcons\n");
-		spin_lock(&cifs_tcp_ses_lock);
-		list_for_each(tmp, &server->smb_ses_list) {
-			ses = list_entry(tmp, struct cifs_ses, smb_ses_list);
-			list_for_each(tmp2, &ses->tcon_list) {
-				tcon = list_entry(tmp2, struct cifs_tcon,
-						  tcon_list);
-				/* add check for persistent handle reconnect */
-				if (tcon && tcon->need_reconnect) {
-					spin_unlock(&cifs_tcp_ses_lock);
-					rc = smb2_reconnect(SMB2_ECHO, tcon);
-					spin_lock(&cifs_tcp_ses_lock);
-				}
-			}
-		}
-		spin_unlock(&cifs_tcp_ses_lock);
+		/* No need to send echo on newly established connections */
+		queue_delayed_work(cifsiod_wq, &server->reconnect, 0);
+		return rc;
 	}
 
-	/* if no session, renegotiate failed above */
-	if (server->tcpStatus == CifsNeedNegotiate)
-		return -EIO;
-
 	rc = small_smb2_init(SMB2_ECHO, NULL, (void **)&req);
 	if (rc)
 		return rc;