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Date: Wed, 4 May 2016 12:42:25 +0300
From: Solar Designer <>
Cc: David Moreno Montero <>,
	Zachary Grafton <>,
	Remi Birot-Delrue <>
Subject: libonion 0.8 contains security fixes


onion is "C library to create simple HTTP servers and Web Applications."

Version 0.8 was released recently:

Changes in this version, as well as in the master branch, include some
security fixes.

In particular, when the library is built with Linux epoll support and is
used in the pool of threads mode, as invoked with onion_new(O_POOL),
there was a file descriptor and data race condition between handling of
epoll events and timeout events.  The code is using EPOLLONESHOT to
avoid epoll event vs. epoll event races, but it is detecting and
handling timeouts manually, operating on the same file descriptors and
shared data structures (connection slots) that epoll operates on.  It's
only after I figured this out, with some help from David Moreno Montero
(onion upstream), and came up with a workaround, based on Rich Felker's
idea, that I found this had previously been observed and reported as a
GitHub issue by MoZhonghua on July 30, 2015:

Apparently, the issue was easily unintentionally triggerable, without
even a stress-test, with onion's default hard-coded epoll_wait()
maxevents of 10.  I was using maxevents of 1 for unrelated reasons (and
this has since become the default), which made the problem much harder
to trigger: it took over 30 million HTTP requests at 10k requests/second
for me to trigger the problem in my stress-testing of an onion-using
application.  However, this very first crash I triggered, running the
application under gdb, resulted in attempted execution of a portion of
an HTTP request or response as code: the freed memory where a connection
slot structure had been was reused in such a way that a data pointer
appeared just where the handler function pointer had been.  On another
occasion, with a later upstream revision with a partial workaround in
place, I had it crash on attempting to make the handler function call
over the pointer, which was directly replaced with data.  The latter
condition was more likely exploitable on modern systems (where the
malloc()'ed HTTP request and response data is non-executable, so need to
use borrowed rather than injected code).

A mitigating factor is that unless a given application (or a wrapper)
has respawn functionality, there's only one chance to try exploiting the
bug.  Another mitigating factor is that epoll, especially with the
specific features used by the code, only exists on modern Linux (so
having this code run on a system with executable malloc()'ed data and
without ASLR is unlikely).

This issue is currently believed to be worked around for good with this
pull request, included in 0.8:

The important commits are:

commit 4e111f30c1adf0ba0d5814a24f904dea35310a37
Author: Solar Designer <>
Date:   Sun Mar 27 19:16:04 2016 +0300

    Complete the implementation of Rich Felker's idea (partially introduced
    with commit c80c46d5ff842291f0cce3917e7b8340c43d4315) to shutdown()
    rather than close() on timeout, so that the fd is held until after
    another one-shot epoll event arrives.  This way, we don't close the fd
    (thereby not freeing it for possible reuse just yet) and don't free the
    slot asynchronously to a possible event for the same slot on another
    thread.  When we do receive another event for the shutdown() fd, we
    expect that no concurrent event is being processed for the same fd and
    the same slot due to the one-shot property of the epoll instance.
    In other words, we postpone the potential fd and slot memory reuse until
    after we're out of the asynchronous timeout handling and into the
    per-slot synchronous one-shot event handling.

    Handle timeouts in busy servers more optimally: check for them once per
    second (and per thread for now) rather than once per event.

commit 0ba14e54ab2c8bee9de68b98db084220cccb2234
Author: Solar Designer <>
Date:   Sun Mar 27 19:31:52 2016 +0300

    On the first call to onion_poller_slot_new(), pre-allocate memory for
    all slots based on the maximum number of fd's allowed for the program
    (the currently effective limit for RLIMIT_NOFILE, or 1000000 at most).
    This assumes that the first call to onion_poller_slot_new() doesn't need
    to be MT-safe (it is expected to be for a listening fd).

    1. Reduced impact of premature slot memory reuse bugs (now accessing a
    reused slot rather than arbitrarily reused memory) in case any are left
    or are reintroduced later or a new kernel version doesn't fully enforce
    the epoll one-shot property.
    2. Performance improvement (one calloc()/free() less per connection).

I've just assigned OVE-20160504-0004 to the onion pre-0.8 epoll event
vs. timeout handling race conditions.

There may still be issues with fd's getting closed on error by other
parts of onion, which would trigger races in the poller again.  In that
case, the additional change to memory allocation as described above
should help reduce the impact.  Once identified, these issues would need
their own fixes (and their own tracking IDs, if anyone cares).

Another likely security issue fixed in 0.8 was found and fixed by
Zachary Grafton:

commit 2e54d367b804f8a803c700065b17edd70afdf615
Author: Zachary Grafton <>
Date:   Tue Mar 15 08:57:28 2016 -0400

    Fix issue with html quoting and a buffer overflow.

    We were incorrectly calculating the size of the new string, &amp; is 5
    characters and not 4. This creates a buffer overflow condition in

I've just assigned OVE-20160504-0005 to this issue.

There's also, with unclear security relevance/impact, but potentially
just as bad:

commit c4f137f2d4c0b09657ae105ae6f133e9cec481a1
Author: Remi Birot-Delrue <>
Date:   Sat Apr 16 18:20:04 2016 +0200

    Fix bugs in onion_response_vprintf.

    For input (format string + arguments) larger than 512 bytes:
    - A null byte was written at the end of input;
    - the variadic list was used a second time without initialization, causing segfaults.

I've just assigned OVE-20160504-0006 to this issue.

There are probably more.  Basically, if you're using onion, consider
upgrading to 0.8 now.

FWIW, my current opinion of onion is: friendly and helpful upstream,
healthy community, but I wish code quality were much better.

There's a good list of other embeddable HTTP server libraries at:

(scroll down to "Alternatives"), as well as indeed libmicrohttpd itself.


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