My contributions to CPython during 2017 Q2 (part 3)

This is the third part of my contributions to CPython during 2017 Q2 (april, may, june):

  • Security
  • Trick bug: Clang 4.0, dtoa and strict aliasing
  • sigwaitinfo() race condition in test_eintr
  • FreeBSD test_subprocess core dump

Previous reports:


Backport fixes

I am trying to fix all known security fixes in the 6 maintained Python branches: 2.7, 3.3, 3.4, 3.5, 3.6 and master.

I created the website to track these vulnerabilities, especially which Python versions are fixed, to identifiy missing backports.

Python 2.7, 3.5, 3.6 and master are quite good, I am still working on backporting fixes into 3.4 and 3.3. Larry Hastings merged my 3.4 backports and other security fixes, and scheduled a new 3.4.7 release next weeks. Later, I will try to fix Python 3.3 as well, before its end-of-life, scheduled for the end of september.

See the Status of Python branches in the devguide.

libexpat 2.2

Python embeds a copy of libexpat to ease Python compilation on Windows and macOS. It means that we have to remind to upgrade it at each libexpat release. It is especially important when security vulerabilities are fixed in libexpat.

libexpat 2.2 was released at 2016-06-21 and it contains such fixes for vulnerabilities, see: CVE-2016-0718: expat 2.2, bug #537.

Sadly, it took us a few months to upgrade libexpat. I wrote a short shell script to easily upgrade libexpat: recreate the Modules/expat/ directory from a libexpat tarball.

My commit:

bpo-29591: Upgrade Modules/expat to libexpat 2.2 (#2164)

Remove the configuration (Modules/expat/*config.h) of unsupported platforms: Amiga, MacOS Classic on PPC32, Open Watcom.

Remove XML_HAS_SET_HASH_SALT define: it became useless since our local expat copy was upgrade to expat 2.1 (it's now expat 2.2.0).

I upgraded libexpat to 2.2 in Pytohn 2.7, 3.4, 3.5, 3.6 and master branches. I still have a pending pull request for 3.3.

libexpat 2.2.1

Just after I finally upgraded our libexpat copy to 2.2.0... libexpat 2.2.1 was released with new security fixes! See CVE-2017-9233: Expat 2.2.1

Again, I upgraded libexpat to 2.2.1 in all branches (pending: 3.3), see bpo-30694. My commit:

Upgrade expat copy from 2.2.0 to 2.2.1 to get fixes of multiple security vulnerabilities including:

  • CVE-2017-9233 (External entity infinite loop DoS),
  • CVE-2016-9063 (Integer overflow, re-fix),
  • CVE-2016-0718 (Fix regression bugs from 2.2.0's fix to CVE-2016-0718)
  • CVE-2012-0876 (Counter hash flooding with SipHash).

Note: the CVE-2016-5300 (Use os-specific entropy sources like getrandom) doesn't impact Python, since Python already gets entropy from the OS to set the expat secret using XML_SetHashSalt().

urllib splithost() vulnerability

Vulnerability: bpo-30500: urllib connects to a wrong host.

While it was quick to confirm the vulnerability, it was tricky to decide how to properly fix it without breaking backward compatibility. We had too few unit tests, and no obvious definition of the expected behaviour. I contributed to the discussed and to polish the fix:

bpo-30500 commit:

Fix urllib.parse.splithost() to correctly parse fragments. For example, splithost('//') now correctly returns the host, instead of treating as the host in an authentification (login@host).

Fix applied to master, 3.6, 3.5, 3.4 and 2.7; pending pull request for 3.3.

Travis CI

I also wrote a pull request to enable Travis CI and AppVeyor CI on Python 3.3 and 3.4 branches, to test security on CI. These changes are complex and not merged yet, but I am now confident that the CI will be enabled on 3.4!

My PR for Python 3.4: [3.4] Backport CI config from master.

Tricky bug: Clang 4.0, dtoa and strict aliasing

Aha, another funny story about compilers: bpo-30104.

I noticed that the following tests started to fail on the "AMD64 FreeBSD CURRENT Debug 3.x" buildbot:

  • test_cmath
  • test_float
  • test_json
  • test_marshal
  • test_math
  • test_statistics
  • test_strtod

First, I bet on a libc change on FreeBSD. Then, I found that test_strtod fails on FreeBSD using clang 4.0, but pass on FreeBSD using clang 3.8.

I started to bisect the code on Linux using a subset of Python/dtoa.c:

  • Start (integrated in CPython code base): 2,876 lines
  • dtoa2.c (standalone): 2,865 lines
  • dtoa5.c: 50 lines

Extract of dtoa5.c:

typedef union { double d; uint32_t L[2]; } U;

struct Bigint { int wds; };

static double
ratio(struct Bigint *a)
    U da, db;
    int k, ka, kb;
    double r;

    da.d = 1.682;
    ka = 6;
    db.d = 1.0;
    kb = 5;
    k = ka - kb + 32 * (a->wds - 12);
    printf("k=%i\n", k);

    if (k > 0)
        da.L[1] += k * 0x100000;
    else {
        k = -k;
        db.L[1] += k * 0x100000;
    r = da.d / db.d;
    /* r == 3.364 */
    return r;

Even if I had a very short C code (50 lines) reproducing the bug, I was still unable to understand the bug. I read many articles about aliasing, and I still don't understand fully the bug... I suggest you these two good articles:

Anyway, I wanted to report the bug to clang (LLVM), but the LLVM bug tracker was migrating and I was unable to subscribe to get an account!

In the meanwhile, Dimitry Andric, a FreeBSD developer, told me that he got exactly the same clang 4.0 issue with "dtoa.c" in the julia programming language. Two months before I saw the same bug, he already reported the bug to FreeBSD: lang/julia: fails to build with clang 4.0, and to clang: After r280351: if/else blocks incorrectly optimized away?.

The "problem" is that clang developers disagree that it's a bug. In short, the discussion was around the C standard: does clang respect C aliasing rules or not? At the end, clang developers consider that they are right to optimize. To summarize:

It's a bug in the code, not in the compiler

So I made a first change to use the -fno-strict-aliasing flag when Python is compiled with clang:

Python/dtoa.c is not compiled correctly with clang 4.0 and optimization level -O2 or higher, because of an aliasing issue on the double/ULong[2] union.

But this change can make Python slower when compiled on clang, so I was asked to only compile Python/dtoa.c with this flag:

On clang, only compile dtoa.c with -fno-strict-aliasing, use strict aliasing to compile all other C files.

sigwaitinfo() race condition in test_eintr

The tricky test_eintr

When I wrote and implemented the PEP 475, Retry system calls failing with EINTR, I didn't expect so many annoying bugs of the newly written test_eintr unit test. This test calls system calls while sending signals every 100 ms. Usually the test tries to block on a system call during at least 200 ms, to make sure that the syscall was interrupted at least once by a signal, to check that Python correctly retries the interrupted system call.

Since the PEP was implemented, I already fixed many race conditions in test_eintr, but there was still a race condition on the sigwaitinfo() unit test. Sometimes on a few specific buildbots (FreeBSD), the test fails randomly.

First attempt

My first attempt was the bpo-25277, opened at 2015-09-30. I added faulthandler to dump tracebacks if a test hangs longer than 10 minutes. Then I changed the sleep from 200 ms to 2 seconds in the sigwaitinfo() test... just to make the bug less likely, but using a longer sleep doesn't fix the root issue.

Second attempt

My second attempt was the bpo-25868, opened at 2015-12-15. I added a pipe to "synchronize the parent and the child processes", to try to make the sigwaitinfo() test a little bit more reliable. I also reduced the sleep from 2 seconds to 100 ms.

7 minutes after my fix, Martin Panter wrote:

With the pipe, there is still a potential race after the parent writes to the pipe and before sigwaitinfo() is invoked, versus the child sleep() call.

What do you think of my suggestion to block the signal? Then (in theory) it should be robust, rather than relying on timing.

I replied that I wasn't sure that sigwaitinfo() EINTR error was still tested if we make his proposed change.

One month later, Martin wrote a patch but I was unable to take a decision on his change. In september 2016, Martin noticed a new test failure on the FreeBSD 9 buildbot.

Third attempt

My third attempt is the bpo-30320, opened at 2017-05-09. This time, I really wanted to fix all buildbot random failures. Since I was now able to reproduce the bug on my FreeBSD VM, I was able to write a fix but also to check that:

  • sigwaitinfo() and sigtimedwait() fail with EINTR and Python automatically restarts the interrupted syscall
  • I hacked the test file to only run the sigwaitinfo() and sigtimedwait() unit tests. Running the test in a loop doesn't fail: I ran the test during 5 minutes in 10 shells (tests running 10 times in parallel) => no failure, the race condition seems to be gone.

So I pushed my fix:

bpo-30320: test_eintr now uses pthread_sigmask()

Rewrite sigwaitinfo() and sigtimedwait() unit tests for EINTR using pthread_sigmask() to fix a race condition between the child and the parent process.

Remove the pipe which was used as a weak workaround against the race condition.

sigtimedwait() is now tested with a child process sending a signal instead of testing the timeout feature which is more unstable (especially regarding to clock resolution depending on the platform).

To be honest, I wasn't really confident, when I pushed my fix, that blocking the waited signal is the proper fix.

So it took 1 year and 8 months to really find and fix the root bug.

Sadly, while I was working on dozens of other bugs, I completely lost track of Martin's patch, even if I opened the bpo-25868. Sorry Martin for forgotting to review your patch! But when you wrote it, I was unable to test that sigwaitinfo() was still failing with EINTR.

FreeBSD test_subprocess core dump

bpo-30448: During one month, some FreeBSD buildbots was emitting this warning which started to annoy me, since I was trying to fix all buildbots warnings:

Warning -- files was modified by test_subprocess
  Before: []
  After:  ['python.core']

I tried and failed to reproduce the warning on my FreeBSD 11 VM. I also asked a friend to reproduce the bug, but he also failed. I was developping my test.bisect tool and I wanted to get access to a machine to reproduce the bug!

Later, Kubilay Kocak aka koobs gave me access to his FreeBSD buildbots and in a few seconds with my new test.bisect tool, I identified that the test_child_terminated_in_stopped_state() test triggers a deliberate crash, but doesn't disable core dump creation. The fix is simple, use context manager. Thanks koobs for the access!

Maybe only FreeBSD 10 and older dump a core on this specific test, not FreeBSD 11. I don't know why. The test is special, it tests a process which crashs while being traced with ptrace().