nullmailer-smtpd fails to handle SIGPIPE, closes unexpectedly

[schongallam]

Brief Description:

• Nullmailer-smtpd reproducibly exits unexpectedly with value 141 if it attempts to write DATA content to a failed stream.
• This bug is ONLY apparent after the segfault bug (issue reproducible crash when calling nullmailer-smtpd from swaks #95) is patched.

Steps to reproduce (see issue #95):

• Give nullmailer-smtpd a malformed payload, where either the MAIL FROM or the RCPT TO target is invalid
• For example:

HELO a\n
MAIL FROM:<b>\n
RCPT TO:<c>\n
DATA\n
line 1\r\n
\r\n.\r\n

Expected behavior: any of the following.

• nullmailer-smtpd should report an SMTP error message.
• reject invalid MAIL FROM or RCPT TO
• report to ferr that write to stream has failed

Resulting behavior:

/app# nullmailer-smtpd
220 nullmailer-smtpd ready
HELO a
250 2.3.0 OK
MAIL FROM:<b>
250 2.1.0 Sender accepted
RCPT TO:<c@d.e>
250 2.1.5 Recipient accepted
DATA
354 End your message with a period on a line by itself
line 1
/app# echo $?
141

(141 = 128 + SIGPIPE)

Root cause:
Running in gdb shows that src/smtpd.cc, qwrite attempts to write to a closed file descriptor (line 118). Verified with signal handling patch (see fix below).

Impact:

• SMTP clients are able to cause failure of nullmailer-smtpd.
• nullmailer-smtpd exits without providing useful information why.

Fix(es):
Implementing a signal handler to catch SIGPIPE in src/smtpd.cc avoids a program crash. Here is a hacky proof of concept:

@@ -111,9 +111,16 @@
   return fout;
 }
 
+static void pipe_handler(const int signal_num)
+{
+  ferr << "signal " << signal_num << ".\n"; 
+  respond(resp_qwrite_err);
+}
+
 static bool qwrite(int qfd, const char* data, size_t len)
 {
   ssize_t wr;
+  signal(SIGPIPE, pipe_handler);
   while (len > 0) {
     wr = write(qfd, data, len);
     if (wr <= 0)

Resulting behavior, avoiding a crash:

/app# nullmailer-smtpd
220 nullmailer-smtpd ready
HELO me
250 2.3.0 OK
MAIL FROM:<a>
250 2.1.0 Sender accepted
RCPT TO:<x@y.z>
250 2.1.5 Recipient accepted
DATA
354 End your message with a period on a line by itself
line 1
451 4.3.0 Write to nullmailer-queue failed
451 4.3.0 Write to nullmailer-queue failed
signal 13.
nullmailer-smtpd: nullmailer-queue failed: 1
QUIT
221 2.0.0 Good bye
``