Committed CVE-2014-7817 to 2.20.

[Carlos O'Donell]

I've committed the fix for CVE-2014-7817 to
2.20. If anyone wants to backport this
to older branches, then please be my guest. It's
very self-contained and should apply cleanly
across lots of older branches.

commit a39208bd7fb76c1b01c127b4c61f9bfd915bfe7c
Author: Carlos O'Donell <carlos@redhat.com>
Date:   Wed Nov 19 11:44:12 2014 -0500

    CVE-2014-7817: wordexp fails to honour WRDE_NOCMD.
    
    The function wordexp() fails to properly handle the WRDE_NOCMD
    flag when processing arithmetic inputs in the form of "$((... ``))"
    where "..." can be anything valid. The backticks in the arithmetic
    epxression are evaluated by in a shell even if WRDE_NOCMD forbade
    command substitution. This allows an attacker to attempt to pass
    dangerous commands via constructs of the above form, and bypass
    the WRDE_NOCMD flag. This patch fixes this by checking for WRDE_NOCMD
    in exec_comm(), the only place that can execute a shell. All other
    checks for WRDE_NOCMD are superfluous and removed.
    
    We expand the testsuite and add 3 new regression tests of roughly
    the same form but with a couple of nested levels.
    
    On top of the 3 new tests we add fork validation to the WRDE_NOCMD
    testing. If any forks are detected during the execution of a wordexp()
    call with WRDE_NOCMD, the test is marked as failed. This is slightly
    heuristic since vfork might be used in the future, but it provides a
    higher level of assurance that no shells were executed as part of
    command substitution with WRDE_NOCMD in effect. In addition it doesn't
    require libpthread or libdl, instead we use the public implementation
    namespace function __register_atfork (already part of the public ABI
    for libpthread).
    
    Tested on x86_64 with no regressions.


Cheers,
Carlos.