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dtrace: failed to grab pid ?!

December 17, 2012

There seems to be a reasonable security feature in Solaris that limits usage of diagnostics tools based on privileges(5) of source and target PIDs. Saying that, tools  such as dtrace(1M) and pstack(1) should have equal or more privileges(5) than the target PID they want to observe. Otherwise the process owner can use the target PID to run instrumented instructions with higher privileges which is obviously a security hole.

But this fair statement can cause some headache especially when processes start from sources other than a shell such as SMF.

Let’s examine this scenario. What prevents the owner of this process to look inside even after setting all dtrace permissions to zone and user.

$ ppriv $$
29763: -bash
flags = <none>
E: basic,dtrace_proc,dtrace_user,proc_owner
I: basic,dtrace_proc,dtrace_user,proc_owner
P: basic,dtrace_proc,dtrace_user,proc_owner
L: basic,contract_event,contract_observer,dtrace_proc,dtrace_user,file_chown,file_chown_self,
$ /usr/ucb/ps auxwww | grep java | fgrep "XX:+ExtendedDTraceProbes" | awk '{print $1,$2}'
weblogic 1745
weblogic 27317
$ pfexec dtrace -n 'hotspot$target:::object-alloc{ @ = quantize(arg1) }' -p 1745
dtrace: failed to grab pid 1745: permission denied
$ pstack 1745
pstack: cannot examine 1745: permission denied

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What’s up? The reason is hiding somewhere in the SMF service manifest. Let’s have a look:

<exec_method name='start' type='method' exec=…>
<method_credential user='weblogic' privileges='basic,sys_resource,…'/>

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Looking at the PID of the service we notice it has extra sys_resource privilege assigned via SMF that we don’t have in our bash PID ($$).

$ ppriv 1745
flags = <none>
E: basic,sys_resource,…
I: basic,sys_resource,…
P: basic,sys_resource,…

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That’s preventing bash PID to access SMF started service PID although they are owned by the same user. So what is this extra privilege?

$ ppriv -lv sys_resource
Allows a process to modify the resource limits specified
by setrlimit(2) and setrctl(2) without restriction.
Allows a process to exceed the per-user maximum number of processes.
Allows a process to extend or create files on a filesystem that
has less than minfree space in reserve.

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That’s a necessary  for this service. So have to add the missing to the user:

usermod -K defaultpriv=basic,sys_resource,… weblogic

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Next time bash has sys_resource privilege and executes dtrace/pstack against SMF service successfully:

$ ppriv $$
8630: -bash
flags = <none>
E: basic,dtrace_proc,dtrace_user,proc_owner,sys_resource
I: basic,dtrace_proc,dtrace_user,proc_owner,sys_resource
P: basic,dtrace_proc,dtrace_user,proc_owner,sys_resource
L: basic,contract_event,contract_observer,dtrace_proc,dtrace_user,file_chown,file_chown_self,file_dac_execute,file_dac_read,file_dac_search,file_dac_write,file_owner,file_setid,ipc_dac_read,ipc_dac_write,ipc_owner,net_bindmlp,net_icmpaccess,net_mac_aware,net_privaddr,net_rawaccess,proc_audit,proc_chroot,proc_lock_memory,proc_owner,proc_setid,proc_taskid,sys_acct,sys_admin,sys_audit,sys_mount,sys_nfs,sys_resource
$ pfexec dtrace -n 'hotspot$target:::object-alloc{ @ = quantize(arg1) }' -p 1745
dtrace: description 'hotspot$target:::object-alloc' matched 1 probe
value ————- Distribution ————- count
536870912 | 0
1073741824 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 8945
2147483648 | 0

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From → dtrace, solaris

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