From: Andy Lutomirski <luto@amacapital.net>
To: Ben Hutchings <ben@decadent.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Stephen Smalley <sds@tycho.nsa.gov>,
Andrew Morton <akpm@linux-foundation.org>,
"security@kernel.org" <security@kernel.org>,
James Morris <james.l.morris@oracle.com>,
Janis Danisevskis <jdanis@google.com>,
Casey Schaufler <casey@schaufler-ca.com>,
Roland McGrath <roland@hack.frob.com>,
Kees Cook <keescook@chromium.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
LSM List <linux-security-module@vger.kernel.org>,
"Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jann@thejh.net>,
"Eric . Biederman" <ebiederm@xmission.com>,
Paul Moore <aul@paul-moore.com>,
Linux FS Devel <linux-fsdevel@vger.kernel.org>,
Oleg Nesterov <oleg@redhat.com>,
Benjamin LaHaise <bcrl@kvack.org>,
Eric Paris <eparis@parisplace.org>,
Seth Forshee <seth.forshee@canonical.com>,
John Johansen <john.johansen@canonical.com>
Subject: Re: [PATCH 2/9] exec: turn self_exec_id into self_privunit_id
Date: Sun, 18 Sep 2016 12:57:46 -0700 [thread overview]
Message-ID: <CALCETrUzgWzjeAoearJCermuoLbrrMEA15C=xgVXVkLsUgWLYA@mail.gmail.com> (raw)
In-Reply-To: <20160918184507.GT10601@decadent.org.uk>
On Sep 18, 2016 8:45 AM, "Ben Hutchings" <ben@decadent.org.uk> wrote:
>
> On Sun, Sep 18, 2016 at 08:31:37PM +0200, Jann Horn wrote:
> > On Sun, Sep 18, 2016 at 07:13:27PM +0100, Ben Hutchings wrote:
> > > On Sun, 2016-09-18 at 17:05 +0200, Jann Horn wrote:
> > > > This ensures that self_privunit_id ("privilege unit ID") is only shared by
> > > > processes that share the mm_struct and the signal_struct; not just
> > > > spatially, but also temporally. In other words, if you do execve() or
> > > > clone() without CLONE_THREAD, you get a new privunit_id that has never been
> > > > used before.
> > > [...]
> > > > +void increment_privunit_counter(void)
> > > > +{
> > > > + BUILD_BUG_ON(NR_CPUS > (1 << 16));
> > > > + current->self_privunit_id = this_cpu_add_return(exec_counter, NR_CPUS);
> > > > +}
> > > [...]
> > >
> > > This will wrap incorrectly if NR_CPUS is not a power of 2 (which is
> > > unusual but allowed).
> >
> > If this wraps, hell breaks loose permission-wise - processes that have
> > no relationship whatsoever with each other will suddenly be able to ptrace
> > each other.
> >
> > The idea is that it never wraps.
>
> That's what I suspected, but wasn't sure. In that case you can
> initialise each counter to U64_MAX/NR_CPUS*cpu and increment by
> 1 each time, which might be more efficient on some architectures.
>
> > It wraps after (2^64)/NR_CPUS execs or
> > forks on one CPU core. NR_CPUS is bounded to <=2^16, so in the worst case,
> > it wraps after 2^48 execs or forks.
> >
> > On my system with 3.7GHz per core, 2^16 minimal sequential non-thread clone()
> > calls need 1 second system time (and 2 seconds wall clock time, but let's
> > disregard that), so 2^48 non-thread clone() calls should need over 100 years.
> >
> > But I guess both the kernel and machines get faster - if you think the margin
> > might not be future-proof enough (or if you think I measured wrong and it's
> > actually much faster), I guess I could bump this to a 128bit number.
>
> Sequential execution speed isn't likely to get significantly faster so
> with those current numbers this seems to be quite safe.
>
But how big can NR_CPUs get before this gets uncomfortable?
We could do:
struct luid {
u64 count:
unsigned cpu;
};
(LUID = locally unique ID).
IIRC my draft PCID code does something similar to uniquely identify
mms. If I accidentally reused a PCID without a flush, everything
would explode.
--Andy
next prev parent reply other threads:[~2016-09-18 19:58 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-18 15:05 [PATCH 0/9] Various fixes related to ptrace_may_access() Jann Horn
2016-09-18 15:05 ` [PATCH 1/9] exec: introduce cred_guard_light Jann Horn
2016-09-18 15:05 ` [PATCH 2/9] exec: turn self_exec_id into self_privunit_id Jann Horn
2016-09-18 18:13 ` Ben Hutchings
2016-09-18 18:31 ` Jann Horn
2016-09-18 18:45 ` Ben Hutchings
2016-09-18 19:08 ` Jann Horn
2016-09-18 19:57 ` Andy Lutomirski [this message]
2016-09-19 15:31 ` Jann Horn
2016-09-18 15:05 ` [PATCH 3/9] proc: use open()-time creds for ptrace checks Jann Horn
2016-09-19 13:01 ` Stephen Smalley
2016-09-19 14:32 ` Jann Horn
2016-09-19 14:45 ` Stephen Smalley
2016-09-18 15:05 ` [PATCH 4/9] futex: don't leak robust_list pointer Jann Horn
2016-09-18 18:28 ` Ben Hutchings
2016-09-18 18:33 ` Jann Horn
2016-09-18 15:05 ` [PATCH 5/9] proc: lock properly in ptrace_may_access callers Jann Horn
2016-09-18 19:15 ` Jann Horn
2016-09-18 15:05 ` [PATCH 6/9] ptrace: warn on ptrace_may_access without proper locking Jann Horn
2016-09-18 15:05 ` [PATCH 7/9] ptrace: forbid ptrace checks against current_cred() from VFS context Jann Horn
2016-09-18 18:38 ` Ben Hutchings
2016-09-18 18:40 ` Jann Horn
2016-09-18 19:57 ` Andy Lutomirski
2016-09-18 20:38 ` Jann Horn
2016-09-18 20:18 ` Linus Torvalds
2016-09-18 20:52 ` Jann Horn
2016-09-18 15:05 ` [PATCH 8/9] fs/proc: fix attr access check Jann Horn
2016-09-18 15:05 ` [PATCH 9/9] Documentation: add security/ptrace_checks.txt Jann Horn
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