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

[Christian Brauner]

On Wed, Oct 10, 2018 at 02:54:22PM +0200, Christian Brauner wrote:
> On Tue, Oct 09, 2018 at 06:26:47PM +0200, Jann Horn wrote:
> > On Tue, Oct 9, 2018 at 6:20 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > > 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
> > 
> > Huh? Why would you need CAP_SYS_PTRACE for anything here? You can
> > ptrace another process running under your UID just fine, no matter
> > what the namespaces are. I'm not sure what you're saying.
> 
> Sorry, I was out the door yesterday when answering this and was too
> brief. I forgot to mention: /proc/sys/kernel/yama/ptrace_scope. It
> should be enabled by default on nearly all distros and even if not -
> which is an administrators choice - you can usually easily enable it via
> sysctl.
> 
> 1 ("restricted ptrace") [default value]
> When  performing an operation that requires a PTRACE_MODE_ATTACH check,
> the calling process must either have the CAP_SYS_PTRACE capability in
> the user namespace of the target process or it must have a predeâ fined
> relationship with the target process.  By default, the predefined
> relationship is that the target process must be a descendant of the
> caller.
> 
> If you don't have it set you're already susceptible to all kinds of
> other attacks and I'm still not convinced we need to bring out the big
> capable(CAP_SYS_ADMIN) gun here.

That being said, given that Tycho agreed to leave in the native
seccomp() way of retrieving an fd from the task without the sys_admin
restriction [1] which we prefer and if we merge it with aforementioned
feature I care way less about whether or not the restriction is upheld
for ptrace() or not.

[1]: https://lists.linuxfoundation.org/pipermail/containers/2018-October/039553.html