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Re: [libvirt] conftest segfault



On 03/16/2013 11:08 PM, Eric Blake wrote:
On 03/16/2013 02:02 PM, Gene Czarcinski wrote:

In fact, just in typing that, I though of at least one glibc CVE bug in
re_compile_pattern, that still has not been fixed as of the glibc
currently in Fedora 18.  Furthermore, I know that this bug was reported
only recently, and that the version of gnulib used in v1.0.2 did not
check for this bug, while the version in v1.0.3 does (see libvirt commit
d09949e29 for where I updated to newer gnulib, precisely to work around
that glibc CVE).  So sure enough, I looked through my config.log and
found:
Libvirt is immune to that bug, but the CVE still affects other glibc
clients that use re_compile_pattern without being aware of the problems,
so you may want to put some pressure on the right people to fix
https://bugzilla.redhat.com/show_bug.cgi?id=905877.

I did a little testing and the problem begins occurring with this commit:

commit d09949e29386c38443c82a2231240cc1e3954a5d
Author: Eric Blake <eblake redhat com>
Date:   Sat Jan 26 13:41:31 2013 -0700

     maint: update to latest gnulib

     CVE-2013-0242 in glibc's regex() can cause a DoS in any daemon
Bingo - you found the very commit that I was just describing above.

If do a ./autogen with the previous commit, no problem.  Move along to
this commit and "bang" ... the segfault message in syslog.
And as I've already told you, this is GOOD, and will continue to happen
as long as glibc is vulnerable to the CVE.  The segfault in configure is
INTENTIONAL - it is gnulib probing whether glibc is broken, in order to
decide whether to rely on glibc or provide its own alternate regex
implementation.  If glibc coredumps, then gnulib uses its alternate, and
libvirtd is a few kilobytes larger but immune to the glibc bug.  If
glibc does not core dump, then gnulib uses glibc, and libvirtd will be
slightly smaller.  Either way, libvirtd is immune to the glibc CVE,
_because_ the configure test was intentionally trying to tickle a
segfault.  Libvirt 1.0.2, on the other hand, is vulnerable to the glibc
bug, because it was not probing for that particular failure.

So, it appears the this version of gnulib fixes something important but
also causes a segfault when ./configure is run with the new gnulib.
What I do not know is if this segfault has any meaning.
The segfault DOES have meaning - it means that glibc is broken, but that
the brokenness of glibc will not impact the rest of libvirtd.  Quit
worrying about it.

Meanwhile, if you are wondering about glibc, just read
https://bugzilla.redhat.com/show_bug.cgi?id=905877 (which you can easily
find by searching for CVE-2013-0242, the very CVE mentioned in the
commit message that you bisected as being the source where ./configure
first tickles a segfault).  It looks like an updated glibc will be
hitting updates-testing of Fedora 18 early next week, at which point,
the segv during ./configure should disappear.
I thought I had checked bugzilla but I searched the comment fields rather than the subject line for CVW-2013-242 ... I will need to remember to do both next time.

Since libvirt uses gnulib for some/many/most/all of what it needs from libc and gnulib specifically has its own copy/version of regexec.c which has exactly the same patch as the one that goes against glibc, you are correct in saying that libvirt does not suffer from the problem. Maybe something should be done to have configure use gnulib instead of glibc for its tests.

One other little thing ... libvirt has a test named "conftest" ... very confusing in this case. I just may submit a patch to rename that particular test.

Gene


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