On Sun, Aug 05, 2018 at 03:19:45PM -0000, Thomas Deutschmann wrote:
> @Christian: Something is weird with the stable-2.0 patch. It applies
> cleanly, but doesn't compile because it refers to LXC_PROC_PID_FD_LEN
> which doesn't exist and isn't added to the patch, so the code doesn't
> compile. I can fix the build by adding this line in utils.h:
> 
> #define LXC_PROC_PID_FD_LEN (6 + LXC_NUMSTRLEN64 + 4 + LXC_NUMSTRLEN64 +
> 1)

Yeah, this was in my backport queue. I just cherry picked the patch to
support LXC_PROC_PID_FD_LEN to stable-2.0 now:

https://github.com/lxc/lxc/commit/f96f5f3c1341e73ee51c8b49bef4ba571c562d8c

So it should work fine now.

Thanks for the report!
Christian

> 
> -- 
> You received this bug notification because you are a member of Ubuntu
> Container Security team, which is subscribed to the bug report.
> https://bugs.launchpad.net/bugs/1783591
> 
> Title:
>   lxc-user-nic allows unprivileged users to open arbitrary files
> 
> Status in lxc package in Ubuntu:
>   Triaged
> Status in lxc source package in Xenial:
>   Triaged
> Status in lxc source package in Bionic:
>   Triaged
> Status in lxc source package in Cosmic:
>   Triaged
> 
> Bug description:
>   Matthias Gerstner from SUSE reported the following:
> 
>   ```
>   Hello,
> 
>   following the lxc security reporting guidelines [1] I am reporting a
>   finding in the lxc-user-nic setuid binary. I'm encrypting this mail as a
>   best practice and because I found valid GPG keys for all of your
>   adresses. Please find my public key attached to this mail.
> 
>   In the context of an openSUSE security audit of the lxc-user-nic setuid
>   binary [2] (currently private bug) I came across an issue that should be
>   adressed. In the "delete" case the program runs the following piece of
>   code unconditionally with effective uid 0 (from lxc_user_nic.c):
> 
>   ```
>           } else if (request == LXC_USERNIC_DELETE) {
>                   netns_fd = open(args.pid, O_RDONLY);
>                   if (netns_fd < 0) {
>                           usernic_error("Could not open \"%s\": %s\n", args.pid,
>                                         strerror(errno));
>                           exit(EXIT_FAILURE);
>                   }
>           }
>   ```
> 
>   `args.pid` is a user controlled parameter and can be an arbitrary path
>   at the moment. Nothing is done with this file descriptor later on in the
>   program except an attempt at `setns(fd, CLONE_NEWNET)` in
>   `is_privileged_over_netns()`.  Still this allows the unprivileged caller
>   of the setuid binary to achieve the following:
> 
>   - it can test for existence of files normally not accessible to the
>     caller (information leak). Example:
>     ```
>     # this file is existing
>     $ /usr/lib/lxc/lxc-user-nic delete path name /root/.bash_history type bridge nic
>     lxc_user_nic.c: 1017: is_privileged_over_netns: Failed to setns() to network namespace Invalid argument
>     lxc_user_nic.c: 1161: main: Process is not privileged over network namespace
> 
>     # this file is not existing
>     $ /usr/lib/lxc/lxc-user-nic delete path name /root/.zsh_history type bridge nic
>     lxc_user_nic.c: 1130: main: Could not open "/root/.zsh_history": No such file or directory
>     ```
> 
>   - it allows to trigger code paths in the kernel that are normally not
>     accessible to the caller. This can happen when opening special files
>     like character and block devices or files in /proc or /sys. Opening
>     some of these files can cause lock or alloc operations or even more
>     complex things to happen like when opening /dev/ptmx, which causes the
>     allocation of a new master/slave pseudo terminal. Therefore this can
>     lead to DoS like situations or have further unspecified impact.
> 
>   For fixing this I suggest opening the file supplied in `args.pid` only
>   with the permissions of the real user, since this is already done in
>   `is_privileged_over_netns()` anyway. Another approach would be the
>   normalization of the input path and then only allowing a path of the
>   pattern /proc/<pid>/ns/net.
> 
>   [1] https://github.com/lxc/lxc/blob/master/README.md#reporting-security-issues
>   [2] https://bugzilla.suse.com/show_bug.cgi?id=988348
> 
>   Best regards
> 
>   Matthias
>   ```
> 
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