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

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  Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program Tycho Andersen <tycho.andersen@canonical.com> - 2015-09-10 00:40 +0200
    Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program Andy Lutomirski <luto@amacapital.net> - 2015-09-10 01:50 +0200
      Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program Tycho Andersen <tycho.andersen@canonical.com> - 2015-09-10 02:20 +0200
        Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program Andy Lutomirski <luto@amacapital.net> - 2015-09-10 02:50 +0200
          Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program Tycho Andersen <tycho.andersen@canonical.com> - 2015-09-10 03:00 +0200

#1221775 — Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program

On Fri, Sep 04, 2015 at 06:27:27PM -0600, Tycho Andersen wrote:
> On Fri, Sep 04, 2015 at 04:08:53PM -0700, Andy Lutomirski wrote:
> > On Fri, Sep 4, 2015 at 3:28 PM, Tycho Andersen
> > <tycho.andersen@canonical.com> 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@canonical.com> 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@canonical.com> 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.

Here's a thought,

The set I'm currently proposing effectively separates the ref-counting
of the struct seccomp_filter from the struct bpf_prog (by necessity,
since we're referring to filters from fds). What if we went a little
futher, and made a copy of each seccomp_filter on fork(), keeping it
pointed at the same bpf_prog but adding some metadata about how it was
inherited (tsk->seccomp.filter->inheritence_count++ perhaps). This
would still require this change:

> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 9c6bea6..efc3f36 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -239,7 +239,7 @@ static int is_ancestor(struct seccomp_filter *parent,
>  	if (parent == NULL)
>  		return 1;
>  	for (; child; child = child->prev)
> -		if (child == parent)
> +		if (child->prog == parent->prog)
>  			return 1;
>  	return 0;
>  }

to get the ancestry right on restore, but the change would make more
sense given that the seccomp_filter pointers were in fact unique at
this point.

To access it, we can change the current set to instead of iterating on
bpf_prog, to iterate on seccomp_filter, and add a few seccomp
commands, so the whole sequence looks like this:

seccomp_fd = ptrace(PTRACE_SECCOMP_GET_FILTER_FD, pid);
if (seccomp(SECCOMP_MODE_FILTER_EBPF, SECCOMP_EBPF_INHERITANCE, fd) > 0) {
        /* mark this as inherited from parent */
} else {
        bpf_fd = seccomp(SECCOMP_MODE_FILTER_EBPF, SECCOMP_EBPF_GET_FD, seccomp_fd);
        bpf(BPF_PROG_DUMP, &attr, sizeof(attr));
        /* save things as normal */
}

Then we need some way to restore this inheritance_count; the only
thing I can think of right now is to let people specify it when they
add the filter via SECCOMP_EBPF_ADD_FD, but that doesn't seem ideal
since these are potentially unprivileged users (root in their user
ns). However, it's not clear to me how one would abuse it given that
is_ancestor actually checks the bpf_prog pointers.

Tycho
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#1221803

On Wed, Sep 9, 2015 at 3:34 PM, Tycho Andersen
<tycho.andersen@canonical.com> wrote:
> On Fri, Sep 04, 2015 at 06:27:27PM -0600, Tycho Andersen wrote:
>> On Fri, Sep 04, 2015 at 04:08:53PM -0700, Andy Lutomirski wrote:
>> > On Fri, Sep 4, 2015 at 3:28 PM, Tycho Andersen
>> > <tycho.andersen@canonical.com> 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@canonical.com> 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@canonical.com> 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.
>
> Here's a thought,
>
> The set I'm currently proposing effectively separates the ref-counting
> of the struct seccomp_filter from the struct bpf_prog (by necessity,
> since we're referring to filters from fds). What if we went a little
> futher, and made a copy of each seccomp_filter on fork(), keeping it
> pointed at the same bpf_prog but adding some metadata about how it was
> inherited (tsk->seccomp.filter->inheritence_count++ perhaps). This
> would still require this change:

Won't that break the tsync mechanism?

--Andy
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#1221810

On Wed, Sep 09, 2015 at 04:44:24PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 9, 2015 at 3:34 PM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
> >
> > Here's a thought,
> >
> > The set I'm currently proposing effectively separates the ref-counting
> > of the struct seccomp_filter from the struct bpf_prog (by necessity,
> > since we're referring to filters from fds). What if we went a little
> > futher, and made a copy of each seccomp_filter on fork(), keeping it
> > pointed at the same bpf_prog but adding some metadata about how it was
> > inherited (tsk->seccomp.filter->inheritence_count++ perhaps). This
> > would still require this change:
> 
> Won't that break the tsync mechanism?

We'll need the change I posted (is_ancestor comparing the underlying
bpf_prog instead of the seccomp_filter), but then I think it'll work.
I guess we'll need to do some more bookkeeping when we install filters
via TSYNC since each thread would need its own seccomp_filter, and
we'd also have to decide whether a filter installed via TSYNC was
inherited or not.

Am I missing something?

Tycho
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#1221822

On Wed, Sep 9, 2015 at 5:13 PM, Tycho Andersen
<tycho.andersen@canonical.com> wrote:
> On Wed, Sep 09, 2015 at 04:44:24PM -0700, Andy Lutomirski wrote:
>> On Wed, Sep 9, 2015 at 3:34 PM, Tycho Andersen
>> <tycho.andersen@canonical.com> wrote:
>> >
>> > Here's a thought,
>> >
>> > The set I'm currently proposing effectively separates the ref-counting
>> > of the struct seccomp_filter from the struct bpf_prog (by necessity,
>> > since we're referring to filters from fds). What if we went a little
>> > futher, and made a copy of each seccomp_filter on fork(), keeping it
>> > pointed at the same bpf_prog but adding some metadata about how it was
>> > inherited (tsk->seccomp.filter->inheritence_count++ perhaps). This
>> > would still require this change:
>>
>> Won't that break the tsync mechanism?
>
> We'll need the change I posted (is_ancestor comparing the underlying
> bpf_prog instead of the seccomp_filter), but then I think it'll work.
> I guess we'll need to do some more bookkeeping when we install filters
> via TSYNC since each thread would need its own seccomp_filter, and
> we'd also have to decide whether a filter installed via TSYNC was
> inherited or not.
>
> Am I missing something?

Yes.  I don't think that:

int fd = [create an ebpf fd];
if (fork()) {
  /* Process A */
  seccomp(attach fd);
  ...
} else {
  /* Process B */
  seccomp(attach fd);
  ...
}

should result in processes A and B being considered to have the same
seccomp_filter state.  In particular, I eventually want to make the
seccomp_filter state be considerably more interesting than just the
bpf program.

IOW I really do think that seccomp_filter should have identity.

There's another severe problem, I think.  Suppose that ebpf1 and ebpf2
are ebpf fds.  If processes C and D start out with no filters at all,
C attaches ebpf1 and ebpf2, and D attaches just ebpf2, then C and D
are definitely *not* in the same state, and neither is an ancestor of
the other.

--Andy
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#1221831

On Wed, Sep 09, 2015 at 05:44:06PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 9, 2015 at 5:13 PM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
> > On Wed, Sep 09, 2015 at 04:44:24PM -0700, Andy Lutomirski wrote:
> >> On Wed, Sep 9, 2015 at 3:34 PM, Tycho Andersen
> >> <tycho.andersen@canonical.com> wrote:
> >> >
> >> > Here's a thought,
> >> >
> >> > The set I'm currently proposing effectively separates the ref-counting
> >> > of the struct seccomp_filter from the struct bpf_prog (by necessity,
> >> > since we're referring to filters from fds). What if we went a little
> >> > futher, and made a copy of each seccomp_filter on fork(), keeping it
> >> > pointed at the same bpf_prog but adding some metadata about how it was
> >> > inherited (tsk->seccomp.filter->inheritence_count++ perhaps). This
> >> > would still require this change:
> >>
> >> Won't that break the tsync mechanism?
> >
> > We'll need the change I posted (is_ancestor comparing the underlying
> > bpf_prog instead of the seccomp_filter), but then I think it'll work.
> > I guess we'll need to do some more bookkeeping when we install filters
> > via TSYNC since each thread would need its own seccomp_filter, and
> > we'd also have to decide whether a filter installed via TSYNC was
> > inherited or not.
> >
> > Am I missing something?
> 
> Yes.  I don't think that:
> 
> int fd = [create an ebpf fd];
> if (fork()) {
>   /* Process A */
>   seccomp(attach fd);
>   ...
> } else {
>   /* Process B */
>   seccomp(attach fd);
>   ...
> }
> 
> should result in processes A and B being considered to have the same
> seccomp_filter state.  In particular, I eventually want to make the
> seccomp_filter state be considerably more interesting than just the
> bpf program.
> 
> There's another severe problem, I think.  Suppose that ebpf1 and ebpf2
> are ebpf fds.  If processes C and D start out with no filters at all,
> C attaches ebpf1 and ebpf2, and D attaches just ebpf2, then C and D
> are definitely *not* in the same state, and neither is an ancestor of
> the other.

Ah, yes.

> IOW I really do think that seccomp_filter should have identity.

What if we kept a pointer to the seccomp_filter that was inherited on
fork()? Everything "below" that in the tree is not inherited, and
everything above is. Unfortunately, it's not obvious how to restore
this state.

Tycho
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