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

On Tue, Oct 9, 2018 at 6:20 PM Christian Brauner [off-list ref] wrote:

On Tue, Oct 09, 2018 at 05:26:26PM +0200, Jann Horn wrote:

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On Tue, Oct 9, 2018 at 4:09 PM Christian Brauner [off-list ref] wrote:

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On Tue, Oct 09, 2018 at 03:50:53PM +0200, Jann Horn wrote:

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On Tue, Oct 9, 2018 at 3:49 PM Christian Brauner [off-list ref] wrote:

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On Tue, Oct 09, 2018 at 03:36:04PM +0200, Jann Horn wrote:

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On Tue, Oct 9, 2018 at 3:29 PM Christian Brauner [off-list ref] wrote:

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One more thing. Citing from [1]

quoted

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.

quoted

- 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.