Date: Thu, 21 Jan 1999 05:19:24 -0000
From: "D. J. Bernstein" <djb@CR.YP.TO.>
To: BUGTRAQ@NETSPACE.ORG
Subject: NetBSD Security Advisory 1999-001: select(2)/accept(2) race condition in TCP servers
One workaround is to run servers under tcpserver:
http://pobox.com/~djb/ucspi-tcp.html
tcpserver handles a single TCP port with a simple accept() loop.
---Dan
P.S. The same bug shows up in (for example) select()+write() if there's
more than one process writing to the same open file. Non-blocking I/O is
unsatisfactory, as discussed in http://pobox.com/~djb/docs/unixapi.html:
Non-blocking I/O (O_NDELAY/O_NONBLOCK) is a feature of an open file.
This means that a program cannot set non-blocking I/O for open files
shared with other processes, such as stdout; it will screw up other
processes that try writing to the same open file at the same time.
Resulting reliability problem: If a multiplexing program uses select()
to see when it can safely write to a pipe on stdout, and then does the
write, it may end up blocking: another process may have filled the
pipe just after select() returned. One workaround is to schedule
signals to interrupt each I/O operation, but this is unnecessarily
difficult to program.
Vendors should provide ndelay_read() and ndelay_write() syscalls that
do not block.
As John Whittaker says: ``It's UNIX. That doesn't necessarily mean it's
right.''
