Bug 61198

Created attachment 48564 [details]
patch to make sshd daemonize itself properly

Still true with current sshd

It still doesn't seem to pose a problem in the Red Hat cases for two reasons: 
1. Our sshd is normally run from scripts at boot up
2. Our login does close fds so seems to avoid leaks

I still think this is a good idea and good practice
 

Actually, this *has* caused a problem before: rpm used to have a bug where
scripts (%post, %postun, etc.) inherited fds from rpm.  As a result, running
"rpm -Uvh openssh-server*.rpm" could hang, because the openssh-server package
contains a %post script to perform "/sbin/service sshd condrestart", and the new
sshd process wound wind up inheriting a bogus fd.

I agree with the OpenSSH guys, in that in the above situation, the true bug lies
with the program (rpm) that never bothered to close its extra fds before forking
a child.  But I disagree with the OpenSSH guys, in that I believe that *any*
program that is security-sensitive in any way absolutely needs to sanitize the
environment it inherits, regardless of whether that sanitizing winds up hiding a
bug in some other program.  (And the fd table definitely counts as an inherited
environment.)

Some additional info:

http://marc.theaimsgroup.com/?l=bind9-workers&m=103112021703700&w=2

Is this issue still relevant?

Does OpenSSH still fail to properly daemonize itself?  Yes.

Will upstream accept a patch to fix it?  No.  Why?  Because they don't
view it as a potential security problem.  I do.  (There have been
cases in the past where programs could be exploited by filling or
mostly-filling the fd table before exec()ing them.)

Paul Vixie's point that _SC_OPEN_MAX can be a huge number (2**24-1) is
a valid one, but after some thought, I believe the best way to address
that is to first attempt to use /proc/self/fd to obtain the list of
open file descriptors.  If /proc/self/fd is unavailable, then fall
back to doing brute-force close() calls up to the value of _SC_OPEN_MAX.

I can supply a patch to implement the behavior in the previous
paragraph.  The issue for you is to choose what you feel is the lesser
of two evils: maintaining a patch that upstream will never accept, or
tolerating sloppy coding that could lead to a security vulnerability
in the future.

I don't see how could the sshd be exploited by filling it's open fds
table - it's not a setuid binary. The different situation would be if
the sshd would be started from an buggy application as in the comment
#3 and the opened fd was exploited somehow by a connecting client. But
even then it would be the responsibility of the buggy exec'er. 

But I think I could accept a patch which would close the fds from
/proc/self/fd if available (without the brute force fallback).

*** Bug 59382 has been marked as a duplicate of this bug. ***

The problem is that there might be situations in which an attacker could
contrive to have sshd [re]started from a buggy application.  In that case, the
risk isn't so much a buggy exec'er as it is a malicious exec'er.

I actually agree with the OpenSSH guys in that sshd shouldn't coddle buggy
exec'er.  But sshd is an extremely security-sensitive application.  It pays to
be paranoid, even if doing so has the undesired side-effect of coddling buggy
exec'ers.

If you'll accept a patch to close open fds based on /proc information, then I'll
provide it.  But there's no way the OpenSSH guys will ever accept such a patch.

I will consider this for FC5.

This should be fixed upstream.

It's not fixed in 4.7p1, as far as I can tell.  Per comment 9, I doubt the
OpenSSH developers will ever fix this, because they don't think it's a bug.