linux-kernel - Re: [RFC 1/3] seccomp: add a return code to trap to userspace

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Date:   Sun, 4 Feb 2018 20:33:25 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>
Subject: Re: [RFC 1/3] seccomp: add a return code to trap to userspace

On Sun, Feb 4, 2018 at 8:01 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> Hi Andy,
>
> On Sun, Feb 04, 2018 at 05:36:33PM +0000, Andy Lutomirski wrote:
>> > The actual implementation of this is fairly small, although getting the
>> > synchronization right was/is slightly complex. Also worth noting that there
>> > is one race still present:
>> >
>> >   1. a task does a SECCOMP_RET_USER_NOTIF
>> >   2. the userspace handler reads this notification
>> >   3. the task dies
>> >   4. a new task with the same pid starts
>> >   5. this new task does a SECCOMP_RET_USER_NOTIF, gets the same cookie id
>> >      that the previous one did
>> >   6. the userspace handler writes a response
>>
>> I'm slightly confused.  I thought the id was never reused for a given
>> struct seccomp_filter.  (Also, shouldn't the id be u64, not u32?)
>
> Well, what happens when u32/64 overflows? Eventually it will wrap.

I think we can safely assume that u64 won't overflow.  Even if we
processed one user return notification on a given seccomp_filter every
nanosecond (which would be insanely fast), that's 584 years.

>
>> On very quick reading, I have a question.  What happens if a process
>> has two seccomp_filters attached, one of them returns
>> SECCOMP_RET_USER_NOTIF, and the *other* one has a listener?
>
> Good question, in seccomp_run_filters(), the first (lowest, last
> applied) filter who returns SECCOMP_RET_USER_NOTIF is the one that
> gets the notification and the other receives nothing.
>
> I don't really have any reason to prefer this behavior, it's just what
> happened without much thought.

Hmm.  This won't nest right.  Maybe we should just disallow a
user-notification-using filter from being applied if there is already
one in the stack.  Then, if anyone cares about making these things
nest right, they can fix it.