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SubjectRe: RFC: fsyscall
David Drysdale <drysdale@google.com> writes:

> On Wed, Sep 9, 2015 at 1:25 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>> Andy Lutomirski <luto@amacapital.net> writes:
>> > On Tue, Sep 8, 2015 at 4:07 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>> >> Perhaps I had missed it but I don't recall capsicum being able to wrap
>> >> things like reboot(2).
>> >>
>> >
>> > Ah, so you want to be able to grant BPF-defined capabilities :)
>>
>> Pretty much.
>>
>> Where I am focusing is turning Posix capabilities into real
>> capabilities. I would not mind if the functionality was a bit more
>> general. Say to be able to handle things like security labels, or
>> anywhere else you might reasonably be asked can you do X?
>>
>> But I would be happy if we just managed to wrap the Posix capabilities
>> and turned them into real capablilities.
>
> Interesting idea! So kind of like the "object" in question is the root
> role, and the different rights for the corresponding object-capability
> (the file descriptor) are the POSIX capabilities (in the simple case
> at least).
>
> And yes, Capsicum doesn't generally interact with things like reboot(2);
> its checks are on top of any DAC policies rather than instead of them,
> as it's a hybrid rather than a pure object-capability system.
>
>> > Off the top of my head, I think that doing this using a nice IPC
>> > mechanism (which barely exists in Linux, but which seL4 and binder (!)
>> > can do very cleanly) would be simpler and more general, if less
>> > self-contained.
>>
>> Less self-contained becomes a problem when you want to pass them between
>> processes written at different times between different people. If there
>> is something conceptually simple we can implement in the kernel it
>> becomes worth it because that becomes the standard which everyone knows
>> to code to.
>>
>> > (Aside: how on earth does anyone think that replacing binder with
>> > kdbus makes any sense? Binder can pass capabilities, and kdbus can't.
>> > OTOH, maybe Android doesn't use the capability-passing ability.)
>>
>> kdbus has file descriptor passing. Beyond that no comment.
>>
>> >> Which really describes what I am trying to tackle. How do we create an
>> >> object that we can pass between processes that limits what we can do in
>> >> the case of the oddball syscalls that require special privileges.
>> >>
>> >> At the same time I still want the caller to be able to pass in data to
>> >> the system calls being called such as REBOOT_CMD_POWER_OFF versus
>> >> REBOOT_CMD_HALT, while being able to filter it and say you may not pass
>> >> REBOOT_CMD_CAD_OFF.
>> >>
>> >
>> > We could have a conservative whitelist of syscalls for which we allow
>> > this usage. I'm a bit worried that there will be very limited use
>> > cases, given that a lot of use cases will want to follow pointers,
>> > which has TOCTOU problems.
>>
>> Time of check to time of use problems. Interesting point.
>>
>> TOCTOU seems to make filtering of system calls in general much less
>> viable then I had hoped or imagined, and seems to be one of the better
>> arguments I have heard against ioctls.
>
> By the way, Robert Watson (one of the progenitors of Capsicum, as it
> happens) has a nice paper about TOCTOU attacks on syscall interposition
> layers that's a good read:
> http://www.watson.org/~robert/2007woot/
>
> (From this perspective, the limitation that seccomp-bpf programs only
> have access to syscall arguments by-value is actually a help -- the filter
> can't look into user memory, so can't be fooled by having memory
> contents changed underneath it. Of course, if the eBPF stuff ever
> changes that we should watch out...)
>
>> I think the cases I care about are much less likely to have TOCTOU
>> problems than system calls in general, so I still may be ok.
>>
>> However it does seem like past a certain point for good filtering the
>> entire syscall ABI needs to be turned into well defined IPC. Ick!
>
> That's roughly one of Robert's suggestions (section 8.2).
>
>> Sigh. I guess it is about time I dig up the places we call capable.
>> Ugh 1696 places in the kernel.. Even filtering out CAP_SYS_ADMIN and
>> CAP_NET_ADMIN the list is longer than I can easily look at.
>>
>> Still reboot isn't a problem ;)
>>
>> Thinking abou the TOCTOU problems with system call filtering the only
>> general solution I can see is to handle it like the compat syscalls
>> but instead of copying things into a temporary on buffer in userspace
>> we copy the data into a temporary in-kernel buffer (filter the system call)
>> fs = get_fs();
>> set_fs(get_ds());
>> /* Call the system call */
>> set_fs(fs);
>>
>> I don't like the whole set_fs() thing (especially if there is any data
>> we did not manage to copy). But it seems like a good conceptual start.
>
> Doing the copies sounds like it would involve understanding & reproducing
> the memory layouts for every syscall pointer argument, which would be a
> lot of code. Or am I misunderstanding something?

Which is what we have for ioctls and some of the system calls in the
compat case. So it is something that has been done before. However I
am going to leave the TOCTOU mess to another time.

If I assume that anything file descriptor based will need another
mechanism to filter what is allowed on a file descriptor, and as such
will need a different mechanism (capsicum perhaps?). That handily
reduces the problem space, and removes almost all cases where reading
data from userspace is interesting as I am talking about pure system calls.

The list of system calls which are not file descriptor based are listed
below. Most of those don't take weird parameter structures that would
be interesting to filter. So I think my fsyscall idea is conceptually
reasonable. It is not a complete solution for passing someone a well
defined subset you are allowed to do but it is interesting.

Eric

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