On Fri, Sep 11, 2020 at 02:43:15PM +0300, Jarkko Sakkinen wrote:
> On Tue, Sep 08, 2020 at 10:30:33PM -0700, Sean Christopherson wrote:
> > > for (c = 0 ; c < addp.length; c += PAGE_SIZE) {
> > > - if (signal_pending(current)) {
> > > - ret = -EINTR;
> > > + if (c == SGX_MAX_ADD_PAGES_LENGTH || signal_pending(current)) {
> > > + ret = c;
> >
> > I don't have an opinion on returning count vs. EINTR, but I don't see the
> > point in arbitrarily capping the number of pages that can be added in a
> > single ioctl(). It doesn't provide any real protection, e.g. userspace
> > can simply restart the ioctl() with updated offsets and continue spamming
> > EADDs. We are relying on other limits, e.g. memcg, rlimits, etc... to
> > reign in malicious/broken userspace.
> >
> > There is nothing inherently dangerous about spending time in the kernel so
> > long as appropriate checks are made, e.g. for a pending signel and resched.
> > If we're missing checks, adding an arbitrary limit won't fix the underlying
> > problem, at least not in a deterministic way.
> >
> > If we really want a limit of some form, adding a knob to control the max
> > size of an enclave seems like the way to go. But even that is of dubious
> > value as I'd rather rely on existing limits for virtual and physical memory,
> > and add a proper EPC cgroup to account and limit EPC memory.
>
> It is better to have a contract in the API that the number of processed
> pages can be less than given, not unlike in syscalls such as write().
That can be handled by a comment, no? If we want to "enforce" the behavior,
I'd rather bail out of the loop after a random number of pages than have a
completely arbitrary limit. The arbitrary limit will create a contract of
its own and may lead to weird guest implementations.
