(I'm filing this against RHEL7, but I suspect the issue applies to other releases of RHEL/Fedora.)

Description of problem:

If tcpdump is called with the -Z and -w options, unless the argument to the -Z option is "root", tcpdump will attempt to chown the capture file specified by the -w option to the user specified with the -Z option. (It takes this action even if the file is already owned by the user specified with -Z.)

However, even if the capture file has the netutils_tmp_t file context, SELinux denies tcpdump (that is, the netutils_t type) the ability to chown the capture file. This causes tcpdump to immediately fail and exit.

Similarly, tcpdump will call setattr() on a pre-existing capture file if the mode of the capture file isn't already what tcpdump expects. The call to setattr() also is denied by SELinux.

(Most people won't see these issues, because most people run tcpdump from interactive sessions that are in unconfined_t. But we are attempting to run tcpdump in an automated fashion via systemd, which is how we encountered the problem.)

Version-Release number of selected component (if applicable):

0:selinux-policy-targeted-3.12.1-153.el7_0.13.noarch

How reproducible:

Call tcpdump with "-Z tcpdump -w capturefile". (If your login is in the unconfined context, you will need to use runcon to ensure tcpdump runs in netutils_t.)

Steps to Reproduce:

$ cd /tmp
$ id
uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
$ rm -f capturefile
$ runcon -t netutils_t tcpdump -i lo -Z tcpdump -w capturefile
tcpdump: Couldn't change ownership of savefile
$ rm capturefile
$ runcon -t netutils_t tcpdump -i lo -Z root -w foo
tcpdump: listening on lo, link-type EN10MB (Ethernet), capture size 65535 bytes
$ ausearch -m avc -ts today | tail -3
time->Tue Jan 20 16:29:23 2015
type=SYSCALL msg=audit(1421789363.087:1155): arch=c000003e syscall=92 success=no exit=-1 a0=b93db0 a1=48 a2=48 a3=7fff9de092f0 items=0 ppid=24402 pid=24509 auid=20576 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts2 ses=89 comm="tcpdump" exe="/usr/sbin/tcpdump" subj=unconfined_u:unconfined_r:netutils_t:s0-s0:c0.c1023 key=(null)
type=AVC msg=audit(1421789363.087:1155): avc:  denied  { chown } for  pid=24509 comm="tcpdump" capability=0  scontext=unconfined_u:unconfined_r:netutils_t:s0-s0:c0.c1023 tcontext=unconfined_u:unconfined_r:netutils_t:s0-s0:c0.c1023 tclass=capability

Additional info:

As long as tcpdump will call chown/setattr, the netutils_t type should be permitted to call (chown, setattr) on files in the (initrc_tmp_t, mnt_t, netutils_tmp_t, tmp_t) context.