tls: clear data after a starttls acknowledgement

After a starttls acknowledgement message, clear the buffers of any
incoming data / commands.  This will ensure that all future data is
handled securely.

Co-authored-by: Pietro Cerutti <gahr@gahr.ch>

conn/socket.c

@@ -301,3 +301,23 @@ struct Connection *mutt_socket_new(enum ConnectionType type)
 
   return conn;
 }
+
+/**
+ * mutt_socket_empty - Clear out any queued data
+ *
+ * The internal buffer is emptied and any data that has already arrived at this
+ * machine (in kernel buffers) is read and dropped.
+ */
+void mutt_socket_empty(struct Connection *conn)
+{
+  if (!conn)
+    return;
+
+  char buf[1024];
+  int bytes;
+
+  while ((bytes = mutt_socket_poll(conn, 0)) > 0)
+  {
+    mutt_socket_read(conn, buf, MIN(bytes, sizeof(buf)));
+  }
+}

conn/socket.h

@@ -39,6 +39,7 @@ enum ConnectionType
 };
 
 int                mutt_socket_close   (struct Connection *conn);
+void               mutt_socket_empty   (struct Connection *conn);
 struct Connection *mutt_socket_new     (enum ConnectionType type);
 int                mutt_socket_open    (struct Connection *conn);
 int                mutt_socket_poll    (struct Connection *conn, time_t wait_secs);

imap/command.c

@@ -1250,6 +1250,13 @@ int imap_exec(struct ImapAccountData *adata, const char *cmdstr, ImapCmdFlags fl
 {
   int rc;
 
+  if (flags & IMAP_CMD_SINGLE)
+  {
+    // Process any existing commands
+    if (adata->nextcmd != adata->lastcmd)
+      imap_exec(adata, NULL, IMAP_CMD_POLL);
+  }
+
   rc = cmd_start(adata, cmdstr, flags);
   if (rc < 0)
   {
@@ -1273,6 +1280,9 @@ int imap_exec(struct ImapAccountData *adata, const char *cmdstr, ImapCmdFlags fl
   do
   {
     rc = imap_cmd_step(adata);
+    // The queue is empty, so the single command has been processed
+    if ((flags & IMAP_CMD_SINGLE) && (adata->nextcmd == adata->lastcmd))
+      break;
   } while (rc == IMAP_RES_CONTINUE);
   mutt_sig_allow_interrupt(false);
 

imap/imap.c

@@ -739,7 +739,10 @@ int imap_open_connection(struct ImapAccountData *adata)
       }
       if (ans == MUTT_YES)
       {
-        enum ImapExecResult rc = imap_exec(adata, "STARTTLS", IMAP_CMD_NO_FLAGS);
+        enum ImapExecResult rc = imap_exec(adata, "STARTTLS", IMAP_CMD_SINGLE);
+        // Clear any data after the STARTTLS acknowledgement
+        mutt_socket_empty(adata->conn);
+
         if (rc == IMAP_EXEC_FATAL)
           goto bail;
         if (rc != IMAP_EXEC_ERROR)

imap/private.h

@@ -73,6 +73,7 @@ typedef uint8_t ImapCmdFlags;          ///< Flags for imap_exec(), e.g. #IMAP_CM
 #define IMAP_CMD_PASS        (1 << 0)  ///< Command contains a password. Suppress logging
 #define IMAP_CMD_QUEUE       (1 << 1)  ///< Queue a command, do not execute
 #define IMAP_CMD_POLL        (1 << 2)  ///< Poll the tcp connection before running the imap command
+#define IMAP_CMD_SINGLE      (1 << 3)  ///< Run a single command
 
 /**
  * enum ImapExecResult - imap_exec return code

nntp/nntp.c

@@ -1899,6 +1899,8 @@ int nntp_open_connection(struct NntpAccountData *adata)
       {
         return nntp_connect_error(adata);
       }
+      // Clear any data after the STARTTLS acknowledgement
+      mutt_socket_empty(conn);
       if (!mutt_str_startswith(buf, "382", CASE_MATCH))
       {
         adata->use_tls = 0;

pop/pop_lib.c

@@ -358,6 +358,8 @@ int pop_open_connection(struct PopAccountData *adata)
     {
       mutt_str_strfcpy(buf, "STLS\r\n", sizeof(buf));
       rc = pop_query(adata, buf, sizeof(buf));
+      // Clear any data after the STLS acknowledgement
+      mutt_socket_empty(adata->conn);
       if (rc == -1)
         goto err_conn;
       if (rc != 0)

smtp.c

@@ -719,6 +719,8 @@ static int smtp_open(struct Connection *conn, bool esmtp)
     if (mutt_socket_send(conn, "STARTTLS\r\n") < 0)
       return SMTP_ERR_WRITE;
     rc = smtp_get_resp(conn);
+    // Clear any data after the STARTTLS acknowledgement
+    mutt_socket_empty(conn);
     if (rc != 0)
       return rc;