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2011-08-27 14:28:19

From:  Eric Paris
Subject:  mmap_min_addr and your local LSM (ok, just SELinux)
Date:  Mon, 20 Jul 2009 19:23:43 -0400
Message-ID:  <1248132223.2654.278.camel@localhost>
Archive-link:  ,
Brad Spengler recently pointed out that the SELinux decision on how to handle mmap_min_addr in some ways weakens system security vs on a system without SELinux (and in other ways can be stronger). There is a trade off and a reason I did what I did but I would like ideas and discussion on how to get the best of both worlds.
 With SELinux mapping the 0 page requires an SELinux policy permission, mmap_zero. Without SELinux mapping the 0 page requires CAP_SYS_RAWIO. Note that CAP_SYS_RAWIO roughly translates to uid=0 since noone really does interesting things with capabilities. The main problem is WINE. I'm told that WINE needs to map the 0 page to support 16bit applications. On distros without SELinux users must disable the mmap_min_addr protections for the ENTIRE system if they want to run WINE.

I believe (from reading mailing lists) if you install WINE on ubuntu it automatically disables these protections. Thus installing wine on ubuntu disables ALL hardening gains of the mmap_min_addr. On Fedora, with SELinux, we allow users to run WINE in a domain that has the SELinux mmap_zero permission and thus other programs/domains, do not have security weakened. Your daemons, like the web server, are still unable to map the 0 page. This is different than distros without SELinux, remember they have to disable protection globally.
But logged in users (by default), under SELinux, are 'unconfined' and can by their very nature run their program in a domain that allows mmap_zero. Trying to 'confine' the 'unconfined' user with SELinux is an open problem which we don't currently even reasonably attempt address on a broad scale. It's like besieging the user in a gentle mist of water hoping they won't try to escape. So in Fedora your web server is a harder entry point to exploit kernel NULL pointer bugs, but you have no protections against a malicious user.
On Ubuntu if you install WINE your web server and your logged in users have no hardening. If you do not install WINE non-root is hardened, anything running as root is not (aka suid apps, aka pulseaudio). So I was thinking today, wondering how to get the best (or at least better) of both worlds on an SELinux system. I was considering adding a second mmap_min_addr_lsm which would typically be equal to mmap_min_addr. The purpose would be to allow the sysadmin to individually control DAC/LSM protections.
The security checks would turn (sort of) into
 if (addr < mmap_min_addr)
 ret |= capable(CAP_SYS_RAWIO);
if (addr < mmap_min_addr_lsm)
ret |= [insert LSM check here]
So on a non-SELinux system users would end up with exactly what they have today. if you want to run WINE as a normal user you have to set mmap_min_addr = 0 and then you no longer need CAP_SYS_RAWIO. Not much else we can do if your distro down support fine grained permissions.
On an SELinux system what this lets me do is default to a stricter setup, one in which you have to have both CAP_SYS_RAWIO and the selinux mmap_zero permission. You, out of the box, get protection for both your malicious logged in user and your web server. Then if a user decides to run WINE they would turn down mmap_min_addr. This would remove the requirement that they are root, and leave the system vulnerable to a malicious user, but would still allow SELinux to protect confined domains and daemons.
Does anyone see a better way to let users continue to be users while protecting most people? Yes SELinux is stronger in some areas than without confining the ability to map the 0 page, but as has be rightly pointed out it's foolish an broken that SELinux can weaken any protections.
 -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to
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