Date: Tue, 30 Dec 2014 18:51:36 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Jilles Tjoelker <jilles@stack.nl> Cc: arch@freebsd.org Subject: Re: Disabling ptrace Message-ID: <20141230165136.GK42409@kib.kiev.ua> In-Reply-To: <20141230140709.GA96469@stack.nl> References: <20141230111941.GE42409@kib.kiev.ua> <20141230140709.GA96469@stack.nl>
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On Tue, Dec 30, 2014 at 03:07:10PM +0100, Jilles Tjoelker wrote:
> On Tue, Dec 30, 2014 at 01:19:41PM +0200, Konstantin Belousov wrote:
> > The question about a facility to disable introspection functionality
> > (ptrace etc) for a process was asked several times. The latest query
> > made me actually code the feature. Note that other systems, e.g. Linux
> > and OSX, do have similar facilities.
>
> > Patch is below, it provides two new procctl(2) requests.
> > PROC_TRACE_ENABLE enables or disables tracing. It includes core
> > dumping, ptrace, ktrace, debugging sysctls and hwpmc.
> > PROC_TRACE_STATUS allows to get the tracing state.
>
> > Most interesting question is how should disabling of trace behave
> > with regard of fork and exec. IMO, the right model is to protect
> > access to the _program_ address space, which translates to inheritance
> > of the attribute for fork, and reenabling the tracing on exec.
>
> I agree. I imagine this will be useful for programs like ssh-agent, to
> protect their unlocked key material.
>
> This is also what Linux provides, and it is simpler than this patch:
> prctl(PR_SET_DUMPABLE) lets a process make their issetugid() equivalent
> return true, including preventing tracing by unprivileged users. You
> could call that unification a hack.
Yes, I do not like this. We have nice and proper p_candebug(9) KPI.
>
> > On the other hand, I understand that some users want to inherit the
> > tracing disable on exec, so there are PROC_TRACE_SET_DISABLED and
> > PROC_TRACE_SET_DISABLED_EXEC, the later makes disable to be kept after
> > exec.
>
> This is apparently meant to protect a whole process tree as a hardening
> measure, or instead of PROC_TRACE_SET_DISABLED if it is undesirable to
> modify the program with key material.
Agreed, it could be reinterpreted this way. Do you suggest to change
name for PROC_TRACE_SET_DISABLED_EXEC ? E.g. PROC_TRACE_SET_DISABLED_TREE ?
>
> > Note that it is trivial for root on the host to circumvent the feature.
>
> I'd prefer if root can still trace normally, without needing any hacks.
> Philosophically, FreeBSD should serve the system administrator first and
> only then the application programmer. Also, the debugging facilities may
> be needed to debug FreeBSD itself (e.g. procstat -k), not just the
> application.
This is reasonable. It seems that the only way to enable host root to
use tracing without allowing jail' roots to do the same, is to introduce
new privilege. I changed the p_candebug() chunk to the following:
/* Denied explicitely */
if ((p->p_flag2 & P2_NOTRACE) != 0) {
error = priv_check(td, PRIV_DEBUG_DENIED);
if (error != 0)
return (error);
}
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