On Mon, Nov 16, 2020 at 04:34:23PM -0800, Dave Hansen wrote: > On 11/16/20 9:54 AM, Dave Hansen wrote: > >> ENCLS instructions must be serialized for a given enclave, but holding > >> encl->lock for an entire ioctl() will result in deadlock due to an enclave > >> triggering reclaim on itself. > >> > >> Building an enclave must also be serialized, i.e. userspace can't queue up > >> EADD on multiple threads, because the order in which pages are added to an > >> enclave affects the measurement. In other words, rejecting the ioctl() as > >> opposed to waiting on a lock is also desirable. > > Sounds like we need should follow up with an add-on patch to get some of > > that into a comment. > > Jarkko, first of all, let's rename: > > SGX_ENCL_IOCTL -> SGX_ENCL_IOCTL_LOCK > > If it walks like a duck and quacks like a duck... > > Sean had a good example of examples of how EADD could go wrong with > multiple threads. Were there more good examples we could stick in a > changelog? I seem to recall that there are more than a few SGX > instructions don't even work in parallel and require software > synchronization. Could we get a list or at least a few more good examples?
It does not need a specific example. A cryptographic measurement requires always a strict ordering. Any parallel use is a good example. > I also think we should be much more assertive about multiple ioctl() > callers: > > /* Multi-threaded enclave management is invalid and unsafe: */ > if (test_and_set_bit(SGX_ENCL_IOCTL_LOCK, &encl->flags)) > return -EINVAL; > > -EBUSY is saying "everything is OK, just busy, please try again later." > -EINVAL is saying, "userspace, you screwed up". Yeah, it is invalid use of the API. I'll change it to -EINVAL. > Also, does SGX_ENCL_IOCTL_LOCK provide serialization for anything other > than the *hardware* instructions? I couldn't find much, although: > > encl->attributes_mask |= SGX_ATTR_PROVISIONKEY; > > seems to be lacking any other serialization. > > sgx_encl_create() also seems like it has no other locking and relies on > SGX_ENCL_IOCTL_LOCK for sanity. Yeah, those ioctl's take advantage of the flag. /Jarkko
