Re: Improving FreeBSD-SA-07:01.jail fix [was: HEADS UP: Re: FreeBSD Security Advisory FreeBSD-SA-07:01.jail]

On Sat, Jan 20, 2007 at 03:24:23PM +0100, Alexander Leidinger wrote:
Quoting Pawel Jakub Dawidek <pjd@xxxxxxxxxxx> (Sat, 20 Jan 2007 14:03:08 +0100):

I fully agree that console.log should be outside a jail. At least noone
proposed safe solution so far, which also means it's not an easy fix.

What's unsafe about my proposal? I did had a look at the code now, and
it should work (with minor mods).

Original:
---snip---
_tmp_jail=${_tmp_dir}/jail.$$
eval jail ${_flags} -i ${_rootdir} ${_hostname} \
${_ip} ${_exec_start} > ${_tmp_jail} 2>&1

if [ "$?" -eq 0 ] ; then
_jail_id=$(head -1 ${_tmp_jail})
i=1
while [ true ]; do
eval out=\"\${_exec_afterstart${i}:-''}\"

if [ -z "$out" ]; then
break;
fi

jexec "${_jail_id}" ${out}
i=$((i + 1))
done

echo -n " $_hostname"
tail +2 ${_tmp_jail} >${_consolelog}
echo ${_jail_id} > /var/run/jail_${_jail}.id
---snip---

Pseudocode proposal, not tested (changes prefixed with 'x'):
---snip---
_tmp_jail=${_tmp_dir}/jail.$$
x # assuming safe _consolelog (inside chroot) according
to the
x # previous mails here in the thread
x eval (echo "" ; \
x jail ${_flags} -I /var/run/jail_${_jail}.id \
x ${_rootdir} ${_hostname} {_ip} ${_exec_start}) \
x > ${_consolelog} 2>&1

if [ "$?" -eq 0 ] ; then
x _jail_id=$(cat /var/run/jail_${_jail}.id)
i=1
while [ true ]; do
eval out=\"\${_exec_afterstart${i}:-''}\"

if [ -z "$out" ]; then
break;
fi

jexec "${_jail_id}" ${out}
i=$((i + 1))
done

echo -n " $_hostname"
x
x
---snip---

Repeating my points:
- sanitize the consolelog path like discussed in this thread
- the jail is not running, so nobody can create a link (jail
root within FS space of another jail still prohibited)
- subshell to group echo and jail
- 'echo ""' to make sure the file exists when the jail starts
- (new) additional flag to jail to write a jid file
- redirect to the consolelog, it is still open from the echo
when the jail starts so there's no race

I did test "(echo 1; sleep 60 ; echo 2) >/tmp/test" in /bin/sh, and it
is line buffered, so the above works.

Where's the security problem in the above?

It looks like it may work, but I still find it a bit risky. If sh(1) can
reopen the file under some conditions or someone in the future will
modify sh(1) in that way (because he won't be aware that such a change
may have impact on system security) we will have a security hole.
Chances are small, but I'm not going to be the one who will accept that
change:)

--
Pawel Jakub Dawidek http://www.wheel.pl
pjd@xxxxxxxxxxx http://www.FreeBSD.org
FreeBSD committer Am I Evil? Yes, I Am!

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