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Date:      Tue, 30 Dec 2014 17:58:40 +0200
From:      Konstantin Belousov <kostikbel@gmail.com>
To:        Shawn Webb <lattera@gmail.com>
Cc:        Jilles Tjoelker <jilles@stack.nl>, freebsd-arch@freebsd.org
Subject:   Re: Disabling ptrace
Message-ID:  <20141230155840.GI42409@kib.kiev.ua>
In-Reply-To: <3368390.qHnOScdmzK@shawnwebb-laptop>
References:  <20141230111941.GE42409@kib.kiev.ua> <20141230140709.GA96469@stack.nl> <3368390.qHnOScdmzK@shawnwebb-laptop>

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On Tue, Dec 30, 2014 at 10:38:56AM -0500, Shawn Webb wrote:
> On Tuesday, December 30, 2014 03:07:10 PM 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.
> > 
> > > 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.
> > 
> > > 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.
> 
> It's easy even for non-root to disable or work around ptrace disabling. 
> LD_PRELOAD, nopping out the instructions, dtrace, etc. Note that for SUID 
> applications, such tricks don't work. The point is that such protections are 
> very easily disabled, even by non-root users for non-SUID applications.
Google for 'It rather involved being on the other side of this airtight hatchway'.


> 
> I'm curious what the use case was that brought this up. And why the requester 
> thinks it's actually useful.
> 
> We at HardenedBSD have introduced a ptrace hardening patch that limits those 
> who can use ptrace to a certain group. We've also added hardening around 
> [lin]procfs. I believe those to be effective against ptrace abuse to a greater 
> extent. It doesn't, though, handle dtrace, something we still need to 
> research.
> 
> Thanks,
> 
> Shawn





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