On 2014-09-26 15:00, Benjamin A. Shelton wrote:
On 09/26/2014 10:59 AM, Doug Newgard wrote:
OK, we're finally getting some examples of where the sh symlink could
be used to trigger this exploit. Thank you.
There are samples that have been available for the past 2-3 days, and
there's a fairly steady stream of new information on various sites
(HN, probably Slashdot, among others). It isn't difficult to find, if
you're willing to look, but you do have to sort through the cruft and
the "sky is falling" paranoia.
Yes, it's this paranoia that I've been trying to cut through in this
thread to get people to start discussing things rationally.
@Benjamin A. Shelton: What do you mean you'd support it for
correctness? Bash is POSIX compliant, anything that uses only POSIX sh
should run correctly on Bash. If it doens't, it should be reported
upstream.
I should specify that by correctness (in this case), I mean to say
POSIX-compliant *minus* the bashisms and rather "interesting" behavior
of the bash interpreter, in the sense that I can take a script written
for /bin/sh and plop it down on any system that expects /bin/sh, and
it doesn't perform (or provide) any additional magic. "Simpler" might
also be an appropriate synonym. bash has some very convenient
behaviors, but I'm not *completely* convinced that the additional
features of a user shell should necessarily be exposed to applications
that expect /bin/sh to behave consistently across Unix/Unix-like OSes
(e.g. Apache's APR and others) while providing a rather creative
interpretation of envvars. bash is big.
I submit that the bug in question is *exactly* the sort of behavior in
question and has, in fact, already been sent upstream (that's what
these bug reports are for, correct?). I may be mistaken, but I don't
believe interpreting a special string of characters in envvars as
functions--even when invoked as /bin/sh--is considered POSIX behavior?
Does POSIX even address this? I don't see anything that specifies
such, and I'm inclined to believe it is bash specific [1] (please
point out if I'm mistaken).
POSIX generally says "it must do this", not "it can only do this and
nothing more". As long as you can take any POSIX compliant sh script and
run it on bash, it's compliant. You're wanting it to hide functionality
in certain circumstances, which isn't wrong, but it isn't required. One
way is not more correct than the other.
Now my question for everyone else is, what will people do *WHEN* a bug
is found in dash? Bash is the most tested shell code base we have, and
I don't buy into the fallacy that a smaller code base is inherently
more secure. Or are you simply relying on security through obscurity?
I believe this "shellshock" vulnerability was discovered by a Red Hat
auditor and has been exploitable for about one major version back.
"Most tested" doesn't always mean "more secure." Also, dash is at
least as old as bash [2].
Smaller code bases do in fact have the potential to be more secure
simply by fault of their relative magnitude: Less code makes it more
readily auditable in less time, and less code (all other things being
equal) with fewer features will exhibit fewer bugs. It's a matter of
probability. It's not an absolute, of course: Some software may be
written by more skilled individuals, but as a code base grows to
include more features, the probability that it will contain errors in
its implementation approaches one.
Smaller code bases can have the potential to be more secure, but that
doesn't mean that they are. The shear amount of testing Bash gets run
through being the default shell for so many things would suggest that
it's likely more secure than a code base that doesn't get this testing.
Similarly, I don't see how switching /bin/sh is security through
obscurity; if someone were advocating replacing /bin/sh with (t)csh
then yes, I might agree with that assertion, but replacing it with
another sh implementation is not. There are only so many sh-compatible
implementations available (and only so many licensed in a manner that
GPL-licensed projects find palatable), so the limited selection most
certainly is not compatible with such a charge.
Let's look at security through obscurity. When Apple started making
their comeback, one of the big reasons non-technical people gave over
and over for switching is that OSX didn't have any viruses. As it became
more popular, guess what happened? Simply put, the smaller the install
base, the less motivation there is to break it. Dash has a far smaller
install/user base than Bash, so Bash is a much larger target.
What technical reasons are there against switching out /bin/sh?
Thusfar, I haven't encountered anything particularly noisome (the
ST2's subl launch script being one exception, probably several
others), but there's certainly something lurking in unseen dark
corners. It seems (superficially, at least) that most everything else
is well behaved and asks specifically for /bin/bash where expected.
Should those circumstances where this isn't the case be considered
bugs? I would say "yes," but others might emphatically say "no."
Benjamin
[1]
http://pubs.opengroup.org/onlinepubs/009604599/basedefs/xbd_chap08.html
[2] http://en.wikipedia.org/wiki/Almquist_shell
My technical reason is simple, I don't think the base install should
have to include another shell implementation when one is already
available. If you want to switch /bin/sh on your machine, go for it. I
just don't think having it as the default is a good way to go.
