Re: [PATCH] x86/sgx: Roof the number of pages process in SGX_IOC_ENCLAVE_ADD_PAGES

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Fri, Sep 11, 2020 at 08:51:27AM -0700, Sean Christopherson wrote:
> 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.

I don't understand.

It is already a random number given that also signal can cause this.

/Jarkko