nOn Fri, 11 Sep 2020 10:51:27 -0500, Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> 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 agree with Sean on potential issues with the arbitrary hard coded limit.
Also returning -EINTR is better way to express to user space that
operations are interrupted by signal and can be retried, which is a known
pattern for this kind of situations.
