Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program

[Andy Lutomirski]

On Fri, Sep 4, 2015 at 3:28 PM, Tycho Andersen
<tycho.andersen@xxxxxxxxxxxxx> wrote:
> On Fri, Sep 04, 2015 at 02:48:03PM -0700, Andy Lutomirski wrote:
>> On Fri, Sep 4, 2015 at 1:45 PM, Tycho Andersen
>> <tycho.andersen@xxxxxxxxxxxxx> wrote:
>> > On Fri, Sep 04, 2015 at 01:17:30PM -0700, Kees Cook wrote:
>> >> On Fri, Sep 4, 2015 at 9:04 AM, Tycho Andersen
>> >> <tycho.andersen@xxxxxxxxxxxxx> wrote:
>> >> > This commit adds a way to dump eBPF programs. The initial implementation
>> >> > doesn't support maps, and therefore only allows dumping seccomp ebpf
>> >> > programs which themselves don't currently support maps.
>> >> >
>> >> > We export the GPL bit as well as a unique ID for the program so that
>> >>
>> >> This unique ID appears to be the heap address for the prog. That's a
>> >> huge leak, and should not be done. We don't want to introduce new
>> >> kernel address leaks while we're trying to fix the remaining ones.
>> >> Shouldn't the "unique ID" be the fd itself? I imagine KCMP_FILE
>> >> could be used, for example.
>> >
>> > No; we acquire the fd per process, so if a task installs a filter and
>> > then forks N times, we'll grab N (+1) copies of the filter from N (+1)
>> > different file descriptors. Ideally, we'd have some way to figure out
>> > that these were all the same. Some sort of prog_id is one way,
>> > although there may be others.
>>
>> I disagree a bit.  I think we want the actual hierarchy to be a
>> well-defined thing, because I have plans to make the hierarchy
>> actually do something.  That means that we'll need to have a more
>> exact way to dump the hierarchy than "these two filters are identical"
>> or "these two filters are not identical".
>
> Can you elaborate on what this would look like? I think with the
> "these two filters are the same" primitive (the same in the sense that
> they were inherited during a fork, not just that
> memcmp(filter1->insns, filter2->insns) == 0) you can infer the entire
> hierarchy, however clunky it may be to do so.
>
> Another issue is that KCMP_FILE won't work in this case, as it
> effectively compares the struct file *, which will be different since
> we need to call anon_inode_getfd() for each call of
> ptrace(PTRACE_SECCOMP_GET_FILTER_FD). We could add a KCMP_BPF (or just
> a KCMP_FILE_PRIVATE_DATA, since that's effectively what it would be).
> Does that make sense? [added Cyrill]
>

I don't really know what it would look like.  I think we want a way to
compare struct seccomp_filter pointers.

FWIW, I *hate* kcmp.  It might be worth trying to come up with a less
awful way to do this.  For example, what if we could generate a kcmpfd
such that each kcmpfd contains (internally) a random symmetric key?
We could have a function that would return kernel pointers encrypted
by that key.

Of course, then we need to make sure that no one ever tries to keep a
kcmpfd around long enough that CRIU needs to checkpoint it, because
that's impossible.

Grr.

--Andy
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