From: Jann Horn <jann@thejh.net>
To: Ben Hutchings <ben@decadent.org.uk>
Cc: Oleg Nesterov <oleg@redhat.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Roland McGrath <roland@hack.frob.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>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Thomas Gleixner <tglx@linutronix.de>,
Benjamin LaHaise <bcrl@kvack.org>,
Andy Lutomirski <luto@amacapital.net>,
Linus Torvalds <torvalds@linux-foundation.org>,
Krister Johansen <kjlx@templeofstupid.com>,
linux-fsdevel@vger.kernel.org,
linux-security-module@vger.kernel.org, security@kernel.org
Subject: Re: [PATCH v3 1/8] exec: introduce cred_guard_light
Date: Wed, 2 Nov 2016 22:54:16 +0100 [thread overview]
Message-ID: <20161102215416.GH8196@pc.thejh.net> (raw)
In-Reply-To: <1478122733.29107.1.camel@decadent.org.uk>
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On Wed, Nov 02, 2016 at 03:38:53PM -0600, Ben Hutchings wrote:
> On Wed, 2016-11-02 at 21:50 +0100, Jann Horn wrote:
> > On Wed, Nov 02, 2016 at 07:18:06PM +0100, Oleg Nesterov wrote:
> > > On 10/30, Jann Horn wrote:
> > > >
> > > > This is a new per-threadgroup lock that can often be taken instead of
> > > > cred_guard_mutex and has less deadlock potential. I'm doing this because
> > > > Oleg Nesterov mentioned the potential for deadlocks, in particular if a
> > > > debugged task is stuck in execve, trying to get rid of a ptrace-stopped
> > > > thread, and the debugger attempts to inspect procfs files of the debugged
> > > > task.
> > >
> > >
> > > Yes, but let me repeat that we need to fix this anyway. So I don't really
> > > understand why should we add yet another mutex.
> >
> >
> > execve() only takes the new mutex immediately after de_thread(), so this
> > problem shouldn't occur there. Basically, I think that I'm not making the
> > problem worse with my patches this way.
> >
> > I believe that it should be possible to convert most existing users of the
> > cred_guard_mutex to the new cred_guard_light - exceptions to that that I
> > see are:
> >
> > - PTRACE_ATTACH
> > - SECCOMP_FILTER_FLAG_TSYNC (sets NO_NEW_PRIVS on remote task)
> >
> > Beyond that, conceptually, the new cred_guard_light could also be turned
> > into a read-write mutex to prevent deadlocks between its users (where
> > execve would take it for writing and everyone else would take it for
> > reading), but afaik the kernel doesn't have an implementation of
> > read-write mutexes yet?
> [...]
>
> It does (rw_semaphore)
Aren't those spinlocks? I don't think those would be appropriate here,
considering that the cred_guard_light can be held during filesystem
access during execve().
> but with PREEMPT_RT enabled they become simple
> mutexes. So I don't think we should rely on that to avoid deadlock.
Well, I don't think it's really necessary - as far as I can tell, the
only locking operations that would be executed with the
cred_guard_light held would be taking the sighand lock, filesystem
stuff and LSM stuff.
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next prev parent reply other threads:[~2016-11-02 21:54 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-30 21:46 [PATCH v3 0/8] Various fixes related to ptrace_may_access() Jann Horn
2016-10-30 21:46 ` [PATCH v3 1/8] exec: introduce cred_guard_light Jann Horn
2016-11-02 18:18 ` Oleg Nesterov
2016-11-02 20:50 ` Jann Horn
2016-11-02 21:38 ` Ben Hutchings
2016-11-02 21:54 ` Jann Horn [this message]
2016-11-03 18:12 ` Oleg Nesterov
2016-11-03 21:17 ` Jann Horn
2016-11-04 13:26 ` Eric W. Biederman
2016-11-04 15:00 ` Eric W. Biederman
2016-11-04 18:04 ` Oleg Nesterov
2016-11-04 18:45 ` Oleg Nesterov
2016-11-05 14:56 ` Oleg Nesterov
2016-11-09 0:34 ` Eric W. Biederman
2016-11-16 20:03 ` Eric W. Biederman
2016-11-08 22:02 ` Kees Cook
2016-11-08 22:46 ` Eric W. Biederman
2016-11-08 22:56 ` Benjamin LaHaise
2016-11-08 23:33 ` Eric W. Biederman
2016-10-30 21:46 ` [PATCH v3 2/8] exec: add privunit to task_struct Jann Horn
2016-10-30 21:46 ` [PATCH v3 3/8] proc: use open()-time creds for ptrace checks Jann Horn
2016-10-30 21:46 ` [PATCH v3 4/8] futex: don't leak robust_list pointer Jann Horn
2016-10-30 21:46 ` [PATCH v3 5/8] proc: lock properly in ptrace_may_access callers Jann Horn
2016-10-30 21:46 ` [PATCH v3 6/8] fs/proc: fix attr access check Jann Horn
2016-10-30 21:46 ` [PATCH v3 7/8] proc: fix timerslack_ns handling Jann Horn
2016-10-30 21:46 ` [PATCH v3 8/8] Documentation: add security/ptrace_checks.txt Jann Horn
2016-11-01 23:57 ` [PATCH v3 0/8] Various fixes related to ptrace_may_access() Linus Torvalds
2016-11-02 18:38 ` Oleg Nesterov
2016-11-02 21:40 ` Jann Horn
2016-11-03 19:09 ` Andrew Morton
2016-11-03 20:01 ` Jann Horn
2016-11-04 0:57 ` James Morris
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