Re: Why isn't root allowed to kill X servers?

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Göran Uddeborg wrote:
When an X server hang and blocked the console of a machine earlier
today I realised the policy (selinux-policy-targeted-2.3.7-2.fc5) does
not allow root to kill, as in SIGKILL, X servers.

    time->Mon Oct 16 07:54:31 2006
    type=SYSCALL msg=audit(1160978071.008:499): arch=c000003e syscall=62 success=yes exit=0 a0=8e4 a1=9 a2=9 a3=0 items=0 pid=3236 auid=503 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 comm="kill" exe="/bin/kill" subj=root:system_r:unconfined_t:s0
    type=AVC msg=audit(1160978071.008:499): avc:  denied  { sigkill } for  pid=3236 comm="kill" scontext=root:system_r:unconfined_t:s0 tcontext=system_u:system_r:xdm_t:s0-s0:c0.c255 tclass=process

I suppose this is by design, but I'm curious over the reasoning.  It's
not much a root session cannot do in the targeted policy.  Why is this
singled out as an exception?

(And is there something else I'm supposed to do with an X server that
hangs and don't respond to any other signal?)

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No this is actually a bug. This is caused by the introduction of mcs policy . You are seeing a side effect of using the forth field. Your root account is running as root:system_r:unconfined_t:s0, While the X Server is running as tcontext=system_u:system_r:xdm_t:s0-s0:c0.c255

There is a constraint in policy that basically says the Ts0 can not kill the s0-s0:c0.c255. You are seeing this because you logged in as a normal user and su to root. If you login directly via the console to root you will probably run at s0-s0:c0.c255, and could kill the xserver.

You can change the default login on your machine to the full range by executing

semanage login -m -rs0-s0:c255 __default__

This will allow all users who become root to kill the X Server and any other process running in this range.

You could also execute

semanage login -a -rs0-s0:c255 USERNAME

To just allow you the rights.

Anyways this problem is fixed in FC6 and I hope to have a large back port of policy for FC5 within the next week to fix this problem on FC5.





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