Re: [PATCH v2 8/8] Documentation: add security/ptrace_checks.txt

From: Krister Johansen <kjlx@templeofstupid.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jann Horn <jann@thejh.net>,
	Krister Johansen <kjlx@templeofstupid.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Roland McGrath <roland@hack.frob.com>,
	Oleg Nesterov <oleg@redhat.com>,
	John Johansen <john.johansen@canonical.com>,
	James Morris <james.l.morris@oracle.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Paul Moore <aul@paul-moore.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Eric Paris <eparis@parisplace.org>,
	Casey Schaufler <casey@schaufler-ca.com>,
	Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Janis Danisevskis <jdanis@google.com>,
	Seth Forshee <seth.forshee@canonical.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Benjamin LaHaise <bcrl@kvack.org>,
	Ben Hutchings <ben@decadent.org.uk>,
	Andy Lutomirski <luto@amacapital.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org, security@kernel.org
Subject: Re: [PATCH v2 8/8] Documentation: add security/ptrace_checks.txt
Date: Thu, 3 Nov 2016 13:43:22 -0700	[thread overview]
Message-ID: <20161103204321.GA2940@templeofstupid.com> (raw)
In-Reply-To: <87d1ih5f5f.fsf@xmission.com>

On Sun, Oct 30, 2016 at 11:14:04PM -0500, Eric W. Biederman wrote:
> Jann Horn <jann@thejh.net> writes:
> 
> > On Sat, Oct 01, 2016 at 08:16:00PM -0700, Krister Johansen wrote:
> >> On Fri, Sep 23, 2016 at 10:40:38PM +0200, Jann Horn wrote:
> >> > +=====================
> >> > +FILESYSTEM DEBUG APIS
> >> > +=====================
> >> > +
> >> > +The pid / tgid entries in procfs contain various entries that allow debugging
> >> > +access to a process. Interesting entries are:
> >> > +
> >> > + - auxv permits an ASLR bypass
> >> > + - cwd can permit bypassing filesystem restrictions in some cases
> >> > + - environ can leak secret tokens
> >> > + - fd can permit bypassing filesystem restrictions or leak access to things like
> >> > +   pipes
> >> > + - maps permits an ASLR bypass
> >> > + - mem grants R+W access to process memory
> >> > + - stat permits an ASLR bypass
> >> > +
> >> > +Of these, all use both a normal filesystem DAC check (where the file owner is
> >> > +the process owner for a dumpable process, root for a nondumpable process) and a
> >> > +ptrace_may_access() check; however, the DAC check may be modified, and the
> >> > +ptrace_may_access() is performed under PTRACE_FSCREDS, meaning that instead of
> >> > +the caller's ruid, rgid and permitted capabilities, the fsuid, fsgid and
> >> > +effective capabilities are used, causing the case where a daemon drops its euid
> >> > +prior to accessing a file for the user to be treated correctly for this check.
> >> 
> >> Thanks for writing this up.
> >> 
> >> Is it worth mentioning some of the less obvious aspects of how user
> >> namespaces interact with the filesystem debug APIs?  Of particular note:
> >> a nondumpable process will always be assigned the global root ids.
> >> Checks against capabilities for procfs require that the uid and gid have
> >> a mapping in the current namepsace.   That's enforced through
> >> capable_wrt_inode_uidgid().
> >
> > Yeah, makes sense. Added that. Thanks!
> 
> That will actually be changing for 4.10.  mm->user_ns allows me to use
> the user namespace id 0 if that id is mapped.

I'll be excited to sync up with this change once you land it in 4.10.
There are a bunch of tools that get confused if you run them in a user
namespace but they can't access /proc/[pid]/whatever.

-K