linux-kernel - Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

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Message-Id: <24C1FE9E-1BAB-49EC-B62C-942B945A7163@amacapital.net>
Date:   Mon, 25 Jun 2018 19:00:56 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        containers@...ts.linux-foundation.org,
        Linux API <linux-api@...r.kernel.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>,
        suda.akihiro@....ntt.co.jp, "Tobin C. Harding" <me@...in.cc>
Subject: Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace



> On Jun 25, 2018, at 6:32 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> 
>> On Sat, Jun 23, 2018 at 12:27:43AM +0200, Jann Horn wrote:
>>> On Fri, Jun 22, 2018 at 11:51 PM Kees Cook <keescook@...omium.org> wrote:
>>> 
>>>> On Fri, Jun 22, 2018 at 11:09 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> One possible extra issue: IIRC /proc/.../mem uses FOLL_FORCE, which is not what we want here.
>> 
>> Uuugh, I forgot about that.
>> 
>>>> How about just adding an explicit “read/write the seccomp-trapped task’s memory” primitive?  That should be easier than a “open mem fd” primitive.
>>> 
>>> Uuugh. Can we avoid adding another "read/write remote process memory"
>>> interface? The point of this series was to provide a lightweight
>>> approach to what should normally be possible via the existing
>>> seccomp+ptrace interface. I do like Jann's context idea, but I agree
>>> with Andy: it can't be a handle to /proc/$pid/mem, since it's
>>> FOLL_FORCE. Is there any other kind of process context id we can use
>>> for this instead of pid? There was once an idea of pid-fd but it never
>>> landed... This would let us get rid of the "id" in the structure too.
>>> And if that existed, we could make process_vm_*v() safer too (taking a
>>> pid-fd instead of a pid).
>> 
>> Or make a duplicate of /proc/$pid/mem that only differs in whether it
>> sets FOLL_FORCE? The code is basically already there... something like
>> this:
> 
> But we want more than just memory access, I think. rootfs access, ns
> fds, etc. all seem like they might be useful, and racy to open.
> 
> I guess I see two options: use the existing id and add something to
> seccomp() to ask if it's still valid or independent of this patchset
> add some kind of pid id :\
> 

I think we use the existing id / cookie / whatever and ask seccomp, or new syscalls, to do the requested operation. This is because we know the target task is in a very special stopping point. As a result, a seccomp-specific mechanism can do RCU-less fd modifications against a single-threaded target, can muck with things like struct cred, etc, while a more general interface can’t.

It might be nice to add a syscall with flags such that it could be used on ptrace-stopped targets later on. Something like:

access_remote_task(int fd, u64 id, u32 type, ...)

Where type is 16 bits of “id and fd is from seccomp” and 16 bits of “write memory” or such.