Date: Fri, 18 Feb 2022 17:33:04 +0100 From: Solar Designer <solar@...nwall.com> To: oss-security@...ts.openwall.com Subject: Re: CVE-2021-3997: Uncontrolled recursion in systemd's systemd-tmpfiles Hi, Sorry for commenting so late, but: On Mon, Jan 10, 2022 at 06:08:29PM +0000, Qualys Security Advisory wrote: > - but if systemd-tmpfiles crashes during the "remove" phase, then it > never enters the "create" phase; > > - and it fails to create the files and directories (specified in > /usr/lib/tmpfiles.d/*.conf) that it should create at boot time; > > - for example, on Ubuntu 21.04, systemd-tmpfiles fails to create the > directory /run/lock/subsys; but because /run/lock is world-writable, > attackers can create their own /run/lock/subsys; and because various > legacy packages and daemons write into /run/lock/subsys as root, the > attackers may create arbitrary files via symlinks in /run/lock/subsys. I think the combination of world-writable /run/lock and writes into /run/lock/subsys as root is a vulnerability on its own, independent of any systemd issues. This is a matter of failure modes: it's fail-open, but should be fail-secure. Further, even without writes into /run/lock/subsys, keeping /run/lock world-writable unnecessarily allows for DoS attacks against other not yet started services that would use it. On the Red Hat'ish systems I've just checked /run/lock is mode 755, on Debian and Ubuntu it's mode 1777. The only non-root-owned entry under /run/lock on an Ubuntu system is /run/lock/whoopsie, but that alone does not tell us whether it was possibly created as root (and then chown'ed). Either way, keeping /run/lock as world-writable should be avoided, even if by also changing something in another package. systemd's tmpfiles.d/legacy.conf.in lists /run/lock as mode 755, and /run/lock/subsys as mode 755 too. (Incidentally, on Owl we had /var/lock as mode 755, but /var/lock/subsys as mode 700 with no issues.) So the Debian and Ubuntu /run/lock mode 1777 looks like those distros' shortcoming that they should fix. Alexander
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