Date: Tue, 27 Jul 2021 09:45:50 +0200 From: Matthias Gerstner <mgerstner@...e.de> To: oss-security@...ts.openwall.com Subject: replay-sorcery: CVE-2021-36983: kms service in version 0.6.0 allows local root exploit and other local attack vectors Hello list, ReplaySorcery  upstream version 0.6.0 has introduced new security issues. I already reviewed version 0.5.0 a while ago and found issues in its implementation of a setuid-root program . By now this setuid-root program has been deprecated by upstream and has been replaced by a systemd service running as root called "replay-sorcery-kms" that is supposed to provide the same functionality (opening a DRI device with the ffmpeg library for hardware acceleration support when recording screen contents). # Findings in Version 0.6.0 The upstream author asked me to check up on the security of the new systemd service. The basic idea is that the kms system service opens any DRI devices via the ffmpeg library and passes back the open file descriptors to clients of a UNIX domain socket that the service provides in the system. I have found the following issues: a) The UNIX domain socket is placed into the fixed path /tmp/replace-sorcery/device.sock. /tmp/replay-sorcery is a predictable path in a world writable directory, i.e. any other user in the system can precreate it and thus take control of the directory and its contents. For example by removing the intended device.sock and creating a different socket there, clients in the system will then communicate with other parties than intended. b) The service calls the `kmsChmod()` function for both /tmp/replay-sorcery and /tmp/replay-sorcery/device.sock. This function performs a `chmod(path, 0777)`. Thus a local attacker can stage symlink attacks in both locations. The attack via /tmp/replay-sorcery is thwarted by the Linux kernel's symlink protection. The attack via the socket filename is not, because /tmp/replay-sorcery will not have a sticky bit set. So the attacker only has to win a race condition between the kms service binding the socket and performing the chown() during startup. This allows for a local root exploit achievable every time when the kms service is starting up. c) During receiption of the RSServiceDeviceInfo data structure from a client, the (size_t) deviceLength parameter has no upper limit i.e. clients can cause denial-of-service by causing large memory allocations in the service. d) When accepting client connections the service does not make sure that the client is somehow authorized to access the local display. E.g. by it being a member of a special restricted group, or by it owning a local active graphical session. This could allow unprivileged local processes to access display contents of logged in interactive users. # CVE Assignment I received CVE-2021-36983 from Mitre for the local root exploit in issue b). # Bugfixes To my knowledge there are currently no bugfixes available for this. I recommend neither to use the setuid-root option nor the systemd service until these issues are handled. # Timeline 2021-07-12: Received review request from the upstream author by e-mail. 2021-07-20: I reported these findings to the upstream author. 2021-07-21: I received the CVE from Mitre and offered the upstream author an embargo until 2021-07-26. 2021-07-27: No reply from the upstream author so far, publication of the findings. Cheers Matthias : https://github.com/matanui159/ReplaySorcery : https://www.openwall.com/lists/oss-security/2021/02/10/1 -- Matthias Gerstner <matthias.gerstner@...e.de> Dipl.-Wirtsch.-Inf. (FH), Security Engineer https://www.suse.com/security Phone: +49 911 740 53 290 GPG Key ID: 0x14C405C971923553 SUSE Software Solutions Germany GmbH HRB 36809, AG Nürnberg Geschäftsführer: Felix Imendörffer Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)
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