Date: Thu, 18 Apr 2019 12:43:55 +0200 From: Matthias Gerstner <mgerstner@...e.de> To: oss-security@...ts.openwall.com Subject: Security issues in snapcraft snap-confine set*id binary Hello, a couple of security issues have been found in the snapcraft package manager  during the course of a security review for inclusion of snapcraft in openSUSE . The basis for this review was upstream version 2.37.4. snapcraft uses a setuid/setgid root program called `snap-confine` to setup namespace based containers for snap packages. While the program's code is generally of good quality the following moderate security issues have been found: 1) Up and including to version 2.37.4 the /tmp directory within a snap container was owned by the first user that entered the container. Since snap containers can be used by multiple users at the same time or a privileged program or daemon may run in a container, the security of files and directories in /tmp is compromised. An attacker can remove or replace files and directories belonging to other users within the container's /tmp to achieve an unspecified impact. This issue was recently fixed by upstream via commit . 2) The `sc_join_preserved_ns()` function along with a number of other function remembers the current working directory (CWD) of the calling user outside of the container and attempts to restore this CWD again within the container. The `chdir()` operation to restore the CWD is performed with root privileges within the container and is prone to symlink attacks. Therefore an unprivileged user can enter arbitrary directories within the container. Example PoC: ``` user1 $ snap run --shell opera user1 $ cd /tmp user1 $ umask 077 user1 $ mkdir user1_private user1 $ cd user1_private user1 $ umask 022 user1 $ mkdir subdir user1 $ cd subdir user1 $ echo secret >file user2 $ cd /tmp user2 $ mkdir -p user1_private/subdir user2 $ ln -s /tmp/user1_private/subdir user2_private user2 $ cd user2_private user2 $ snap run --shell opera user2 $ /tmp/user1_private/subdir user2 $ cat file secret ``` In this example user2 enters the public sub-directories of a private tmp directory of user1 within the application container. Basically this can also be used to enter /root or /var/lib/snapd/hostfs/root/.config and so on. The severity of this issue is reduced, because snap-confine employs extensive AppArmor profiles that prevent many file system accesses beyond classic DAC permission checks. This issue is addressed by an upstream PR . 3) In function `sc_parse_mountinfo_entry()` the pseudo file /proc/self/mountinfo is parsed. In the subsequently called helper function `parse_next_string_field()` most of the content of each line from that file is parsed. This helper function uses `sscanf()` with a "%s%n" format. %s in `sscanf()` extracts a whitespace delimited string. While the kernel protects e.g. against newlines found in directory names by encoding them specially in the mountinfo file, there are whitespace characters that are not protected against. Unprivileged users can use mechanisms like FUSE based file systems like sshfs to mount file systems onto such strangely named directories: ``` user $ cd /tmp user $ mkdir `echo -e "strange\rdir"` user $ sshfs localhost:/tmp strange?dir ``` The result will be an entry in /proc/self/mountinfo that looks like this: ``` 111 107 0:49 / /tmp/strange\rdir rw,nosuid,nodev,relatime shared:59 - fuse.sshfs localhost:/tmp/ rw,user_id=1000,group_id=100\n ``` The parsing logic in snap-confine will wrongly treat "/tmp/strange" as the `mount_dir` and continue parsing "the next field" after the carriage return. Basically this allows the attacker to provide the rest of the fields parsed by snap-confine via the directory name. In my tests no failure was detected by `sc_parse_mountinfo()`, since parsing "%s" always works as long there is data left to parse. I don't see any viable attack vector here at the moment. To actually control the behaviour of snap-confine we'd need to control the field (4) of the mountinfo file. This field is only available to unprivileged users if they can perform a bind mount. Maybe there is a setuid tool around that allows such things, then it would be a tool to further influence what snap-confine does. In such a case it could e.g. be used to force triggering of the discard-ns logic of snap-confine. This issue is addressed by an upstream PR . I've asked upstream to assign CVEs for issues 1) and 2) but they're not available yet. Best Regards Matthias : https://snapcraft.io/ : https://bugzilla.suse.com/show_bug.cgi?id=1127368 : https://github.com/snapcore/snapd/commit/bdbfeebef03245176ae0dc323392bb0522a339b1 : https://github.com/snapcore/snapd/pull/6642 : https://github.com/snapcore/snapd/pull/6605 -- 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 Linux GmbH GF: Felix Imendörffer, Mary Higgins, Sri Rasiah HRB 21284 (AG Nuernberg) Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)
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