Re: [PATCH v2 seccomp 3/6] seccomp/cache: Add "emulator" to check if filter is arg-dependent

[Andy Lutomirski]

On Fri, Sep 25, 2020 at 1:37 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Fri, Sep 25, 2020 at 12:51:20PM -0700, Andy Lutomirski wrote:
> >
> >
> > > On Sep 25, 2020, at 12:42 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Sep 25, 2020 at 11:45:05AM -0500, YiFei Zhu wrote:
> > >> On Thu, Sep 24, 2020 at 10:04 PM YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote:
> > >>>> Why do the prepare here instead of during attach? (And note that it
> > >>>> should not be written to fail.)
> > >>>
> > >>> Right.
> > >>
> > >> During attach a spinlock (current->sighand->siglock) is held. Do we
> > >> really want to put the emulator in the "atomic section"?
> > >
> > > It's a good point, but I had some other ideas around it that lead to me
> > > a different conclusion. Here's what I've got in my head:
> > >
> > > I don't view filter attach (nor the siglock) as fastpath: the lock is
> > > rarely contested and the "long time" will only be during filter attach.
> > >
> > > When performing filter emulation, all the syscalls that are already
> > > marked as "must run filter" on the previous filter can be skipped for
> > > the new filter, since it cannot change the outcome, which makes the
> > > emulation step faster.
> > >
> > > The previous filter's bitmap isn't "stable" until siglock is held.
> > >
> > > If we do the emulation step before siglock, we have to always do full
> > > evaluation of all syscalls, and then merge the bitmap during attach.
> > > That means all filters ever attached will take maximal time to perform
> > > emulation.
> > >
> > > I prefer the idea of the emulation step taking advantage of the bitmap
> > > optimization, since the kernel spends less time doing work over the life
> > > of the process tree. It's certainly marginal, but it also lets all the
> > > bitmap manipulation stay in one place (as opposed to being split between
> > > "prepare" and "attach").
> > >
> > > What do you think?
> > >
> > >
> >
> > I’m wondering if we should be much much lazier. We could potentially wait until someone actually tries to do a given syscall before we try to evaluate whether the result is fixed.
>
> That seems like we'd need to track yet another bitmap of "did we emulate
> this yet?" And it means the filter isn't really "done" until you run
> another syscall? eeh, I'm not a fan: it scratches at my desire for
> determinism. ;) Or maybe my implementation imagination is missing
> something?
>

We'd need at least three states per syscall: unknown, always-allow,
and need-to-run-filter.

The downsides are less determinism and a bit of an uglier
implementation.  The upside is that we don't need to loop over all
syscalls at load -- instead the time that each operation takes is
independent of the total number of syscalls on the system.  And we can
entirely avoid, say, evaluating the x32 case until the task tries an
x32 syscall.

I think it's at least worth considering.

--Andy