dash/ash [LWN.net]

dash/ash

Posted Oct 2, 2014 15:54 UTC (Thu) by ibukanov (subscriber, #3942)
In reply to: dash/ash by niner
Parent article: Bash gets shellshocked

> it is much easier to program safely in Perl than in Bash.

I want to repeat that Shellshock has nothing to do with programming style of Bash scripts. It comes from a badly designed and implemented feature of the Bash interpreter that is written in C. Perl runtime could just as easily provide a similar "feature" affecting any Perl script, strict or not. For example, can you with certainty assert that Perl interpreter has no bugs related to reading of environment variables that could trigger execution of arbitrary Perl code?

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dash/ash

Posted Oct 2, 2014 17:31 UTC (Thu) by dskoll (subscriber, #1630) [Link] (3 responses)

Perl runtime could just as easily provide a similar "feature" affecting any Perl script, strict or not.

But it doesn't. Well, with one exception: Setting PERL5DB will make perl execute arbitrary Perl code, but only if it has been invoked with the "-d" command-line flag which says to run under a debugger, and no Perl script uses that flag.

Perl makes environment variables available in the %ENV hash, but certainly doesn't try to interpret them as Perl code (modulo the single exception above.)

dash/ash

Posted Oct 5, 2014 20:15 UTC (Sun) by alankila (guest, #47141) [Link] (2 responses)

Environment variable PERL5INC would allow specifying library paths that Perl will look into first. If an attacker can control files in the system, then he can probably control the Perl interpreter through setting PERL5INC into suitable target path, and then PERL5OPT to load it.

Thankfully, none of this is even close to as bad as what bash did.

dash/ash

Posted Oct 7, 2014 14:35 UTC (Tue) by dskoll (subscriber, #1630) [Link] (1 responses)

PERL5LIB and PERL5INC are not used in taint mode. Bash really needs a taint mode.

dash/ash

Posted Oct 7, 2014 17:31 UTC (Tue) by mathstuf (subscriber, #69389) [Link]

Bash needs an overlay:

 _____________________________________________ 
/ It looks like your script is over 100 lines \
\ long; did you mean to write this in Perl?   /
 --------------------------------------------- 
    \
     \
                                   .::!!!!!!!:.
  .!!!!!:.                        .:!!!!!!!!!!!!
  ~~~~!!!!!!.                 .:!!!!!!!!!UWWW$$$ 
      :$$NWX!!:           .:!!!!!!XUWW$$$$$$$$$P 
      $$$$$##WX!:      .<!!!!UW$$$$"  $$$$$$$$# 
      $$$$$  $$$UX   :!!UW$$$$$$$$$   4$$$$$* 
      ^$$$B  $$$$\     $$$$$$$$$$$$   d$$R" 
        "*$bd$$$$      '*$$$$$$$$$$$o+#" 
             """"          """""""

dash/ash

Posted Oct 2, 2014 21:25 UTC (Thu) by flussence (guest, #85566) [Link] (1 responses)

> Perl runtime could just as easily provide a similar "feature" affecting any Perl script, strict or not.

It's had that feature for decades: 2-arg open() will happily interpret any filename passed to it containing a "|" prefix or suffix to mean a command pipe, and helpfully give the rest of the string to the shell to run. The same function is also used internally to pass filenames in ARGV into the magic <> line-iterator.

dash/ash

Posted Oct 3, 2014 11:23 UTC (Fri) by dskoll (subscriber, #1630) [Link]

2-arg open() will happily interpret any filename passed to it containing a "|" prefix or suffix

That's a little different from the bash bug. It requires the programmer to write a script that doesn't handle user-input safely. It's also stopped in taint mode.

The Bash bug doesn't require any action on the part of the script writer; it happens before your script even has a chance to do anything.