[PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

[Andy Lutomirski]

On Wed, Jun 25, 2014 at 10:57 AM, Kees Cook <keescook at chromium.org> wrote:
> On Wed, Jun 25, 2014 at 10:24 AM, Oleg Nesterov <oleg at redhat.com> wrote:
>> On 06/25, Kees Cook wrote:
>>>
>>> On Wed, Jun 25, 2014 at 9:52 AM, Oleg Nesterov <oleg at redhat.com> wrote:
>>> >
>>> > Yes, at least this should close the race with suid-exec. And there are no
>>> > other users. Except apparmor, and I hope you will check it because I simply
>>> > do not know what it does ;)
>>> >
>>> >> I wonder if changes to nnp need to "flushed" during syscall entry
>>> >> instead of getting updated externally/asynchronously? That way it
>>> >> won't be out of sync with the seccomp mode/filters.
>>> >>
>>> >> Perhaps secure computing needs to check some (maybe seccomp-only)
>>> >> atomic flags and flip on the "real" nnp if found?
>>> >
>>> > Not sure I understand you, could you clarify?
>>>
>>> Instead of having TSYNC change the nnp bit, it can set a new flag, say:
>>>
>>>     task->seccomp.flags |= SECCOMP_NEEDS_NNP;
>>>
>>> This would be set along with seccomp.mode, seccomp.filter, and
>>> TIF_SECCOMP. Then, during the next secure_computing() call that thread
>>> makes, it would check the flag:
>>>
>>>     if (task->seccomp.flags & SECCOMP_NEEDS_NNP)
>>>         task->nnp = 1;
>>>
>>> This means that nnp couldn't change in the middle of a running syscall.
>>
>> Aha, so you were worried about the same thing. Not sure we need this,
>> but at least I understand you and...
>>
>>> Hmmm. Perhaps this doesn't solve anything, though? Perhaps my proposal
>>> above would actually make things worse, since now we'd have a thread
>>> with seccomp set up, and no nnp. If it was in the middle of exec,
>>> we're still causing a problem.
>>
>> Yes ;)
>>
>>> I think we'd also need a way to either delay the seccomp changes, or
>>> to notice this condition during exec. Bleh.
>>
>> Hmm. confused again,
>
> I mean to suggest that the tsync changes would be stored in each
> thread, but somewhere other than the true seccomp struct, but with
> TIF_SECCOMP set. When entering secure_computing(), current would check
> for the "changes to sync", and apply them, then start the syscall. In
> this way, we can never race a syscall (like exec).

I'm not sure that helps.  If you set a pending filter part-way through
exec, and exec copies that pending filter but doesn't notice NNP, then
there's an exploitable race.

>
>>> What actually happens with a multi-threaded process calls exec? I
>>> assume all the other threads are destroyed?
>>
>> Yes. But this is the point-of-no-return, de_thread() is called after the execing
>> thared has already passed (say) check_unsafe_exec().
>>
>> However, do_execve() takes cred_guard_mutex at the start in prepare_bprm_creds()
>> and drops it in install_exec_creds(), so it should solve the problem?
>
> I can't tell yet. I'm still trying to understand the order of
> operations here. It looks like de_thread() takes the sighand lock.
> do_execve_common does:
>
> prepare_bprm_creds (takes cred_guard_mutex)
> check_unsafe_exec (checks nnp to set LSM_UNSAFE_NO_NEW_PRIVS)
> prepare_binprm (handles suid escalation, checks nnp separately)
>     security_bprm_set_creds (checks LSM_UNSAFE_NO_NEW_PRIVS)
> exec_binprm
>     load_elf_binary
>         flush_old_exec
>             de_thread (takes and releases sighand->lock)
>         install_exec_creds (releases cred_guard_mutex)
>
> I don't see a way to use cred_guard_mutex during tsync (which holds
> sighand->lock) without dead-locking. What were you considering here?
>

Grab cred_guard_mutex and then sighand->lock, perhaps?

> -Kees
>
> --
> Kees Cook
> Chrome OS Security



-- 
Andy Lutomirski
AMA Capital Management, LLC