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Re: SSHFS (SELinux) frustrations




I am trying to implement SSH FS on a shared DMZ box running SELinux (the latest policy in FC15) and have the layout almost a carbon copy of what is described in [1]. I have been frustrated by multiple failures, however.

The first thing I did after creating the directories is to set the SELinux domain on these:

/home/sftp-chroot -d gen_context(system_u:object_r:user_home_dir_t,s0) /home/sftp-chroot/home(/.*)? gen_context(system_u:object_r:user_home_dir_t,s0) /home/sftp-chroot/dev -d gen_context(system_u:object_r:device_t,s0) /home/sftp-chroot/dev/log -s gen_context(system_u:object_r:devlog_t,mls_systemhigh)

I've also altered the rsyslog in the way described in [1] so that a log socket can be used inside chroot. I also altered the policy to enable ssh home dirs access - gen_tunable(sftpd_enable_homedirs, true) in ftp.te. On a side note, is there a way to do that in my policy (sftpd_enable_homedirs -> on), without either altering ftp.te or using setsebool? I've also noticed that /usr/libexec/openssh/sftp-server has "bin_t" type - shouldn't that be "sftpd_t"?

When I try to use SSHFS with a dedicated using user account I used to get a lot of AVC's, but I eventually got them reduced to just 2 - one known (below), and one unknown:

type=AVC msg=audit(1326034699.037:356): avc: denied { create } for pid=2713 comm="sshd" name="ltm-0.42.0.tar.bz2" scontext=system_u:system_r:sshd_t:s0-s0:c0.c1023 tcontext=system_u:object_r:user_home_dir_t:s0 tclass=file type=SYSCALL msg=audit(1326034699.037:356): arch=40000003 syscall=5 success=no exit=-13 a0=241388 a1=80c1 a2=81b4 a3=233284 items=0 ppid=2710 pid=2713 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=30 comm="sshd" exe="/usr/sbin/sshd" subj=system_u:system_r:sshd_t:s0-s0:c0.c1023 key=(null)
I fixed this over the weekend, but thought to post a brief note of my solution here for posterity just in case someone else stumbles across the same problem.

As is usually the case with SELinux, I went for the "sledge-hammer" solution after spending quite a number of hours banging my head against the wall - I defined my own type (sshfs_root_t in my case) and also constructed a separate pair of .te & .fc files responsible for SSHFS implementation - sshfs.te & sshfs.fc. In the .fc file I defined the context of the sshfs chroot system to be assigned to my newly-created type - "/home/sftp-chroot(/.*)?". In sshfs.te I used the manage_{dirs,files,sock_files,lnk_files}_pattern macros as well as filetrans_pattern define from file_patterns.spt to grant the appropriate level of access for this new context to the sftp_t and sshd_t types.

It was also necessary to grant similar access for the new context to syslogd_t in order to enable proper login inside the chroot. After including this in my new policy all is well and everything works as it should!

Admittedly, I could have used various tunables present in both ssh.te and ftp.te policies to grant the appropriate access to "user_home_dir_t" as was my initial intention, but this was giving a wider-than-intended access to *all* directories defined by that context - something I was not very happy with, hence I went for the newly-defined context instead, so that I could limit the directories as well as the domains to which to grant such access.

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