linux-kernel - Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

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Message-ID: <20181009162022.d7fd2wibyq6xi6sg@brauner.io>
Date:   Tue, 9 Oct 2018 18:20:24 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Jann Horn <jannh@...gle.com>
Cc:     Tycho Andersen <tycho@...ho.ws>, Kees Cook <keescook@...omium.org>,
        Linux API <linux-api@...r.kernel.org>,
        containers@...ts.linux-foundation.org, suda.akihiro@....ntt.co.jp,
        Oleg Nesterov <oleg@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        selinux@...ho.nsa.gov, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>
Subject: Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from
 ptrace

On Tue, Oct 09, 2018 at 05:26:26PM +0200, Jann Horn wrote:
> On Tue, Oct 9, 2018 at 4:09 PM Christian Brauner <christian@...uner.io> wrote:
> > On Tue, Oct 09, 2018 at 03:50:53PM +0200, Jann Horn wrote:
> > > On Tue, Oct 9, 2018 at 3:49 PM Christian Brauner <christian@...uner.io> wrote:
> > > > On Tue, Oct 09, 2018 at 03:36:04PM +0200, Jann Horn wrote:
> > > > > On Tue, Oct 9, 2018 at 3:29 PM Christian Brauner <christian@...uner.io> wrote:
> > > > > > One more thing. Citing from [1]
> > > > > >
> > > > > > > I think there's a security problem here. Imagine the following scenario:
> > > > > > >
> > > > > > > 1. task A (uid==0) sets up a seccomp filter that uses SECCOMP_RET_USER_NOTIF
> > > > > > > 2. task A forks off a child B
> > > > > > > 3. task B uses setuid(1) to drop its privileges
> > > > > > > 4. task B becomes dumpable again, either via prctl(PR_SET_DUMPABLE, 1)
> > > > > > > or via execve()
> > > > > > > 5. task C (the attacker, uid==1) attaches to task B via ptrace
> > > > > > > 6. task C uses PTRACE_SECCOMP_NEW_LISTENER on task B
> > > > > >
> > > > > > Sorry, to be late to the party but would this really pass
> > > > > > __ptrace_may_access() in ptrace_attach()? It doesn't seem obvious to me
> > > > > > that it would... Doesn't look like it would get past:
> > > > > >
> > > > > >         tcred = __task_cred(task);
> > > > > >         if (uid_eq(caller_uid, tcred->euid) &&
> > > > > >             uid_eq(caller_uid, tcred->suid) &&
> > > > > >             uid_eq(caller_uid, tcred->uid)  &&
> > > > > >             gid_eq(caller_gid, tcred->egid) &&
> > > > > >             gid_eq(caller_gid, tcred->sgid) &&
> > > > > >             gid_eq(caller_gid, tcred->gid))
> > > > > >                 goto ok;
> > > > > >         if (ptrace_has_cap(tcred->user_ns, mode))
> > > > > >                 goto ok;
> > > > > >         rcu_read_unlock();
> > > > > >         return -EPERM;
> > > > > > ok:
> > > > > >         rcu_read_unlock();
> > > > > >         mm = task->mm;
> > > > > >         if (mm &&
> > > > > >             ((get_dumpable(mm) != SUID_DUMP_USER) &&
> > > > > >              !ptrace_has_cap(mm->user_ns, mode)))
> > > > > >             return -EPERM;
> > > > >
> > > > > Which specific check would prevent task C from attaching to task B? If
> > > > > the UIDs match, the first "goto ok" executes; and you're dumpable, so
> > > > > you don't trigger the second "return -EPERM".
> > > >
> > > > You'd also need CAP_SYS_PTRACE in the mm->user_ns which you shouldn't
> > > > have if you did a setuid to an unpriv user. (But I always find that code
> > > > confusing.)
> > >
> > > Only if the target hasn't gone through execve() since setuid().
> >
> > Sorry if I want to know this in excessive detail but I'd like to
> > understand this properly so bear with me :)
> > - If task B has setuid()ed and prctl(PR_SET_DUMPABLE, 1)ed but not
> >   execve()ed then C won't pass ptrace_has_cap(mm->user_ns, mode).
> 
> Yeah.
> 
> > - If task B has setuid()ed, exeved()ed it will get its dumpable flag set
> >   to /proc/sys/fs/suid_dumpable
> 
> Not if you changed all UIDs (e.g. by calling setuid() as root). In
> that case, setup_new_exec() calls "set_dumpable(current->mm,
> SUID_DUMP_USER)".

Actually, looking at this when C is trying to PTRACE_ATTACH to B as an
unprivileged user even if B execve()ed and it is dumpable C still
wouldn't have CAP_SYS_PTRACE in the mm->user_ns unless it already is
privileged over mm->user_ns which means it must be in an ancestor
user_ns.

> 
> > which by default is 0. So C won't pass
> >   (get_dumpable(mm) != SUID_DUMP_USER).
> > In both cases PTRACE_ATTACH shouldn't work. Now, if
> > /proc/sys/fs/suid_dumpable is 1 I'd find it acceptable for this to work.
> > This is an administrator choice.