On Fri, 04 Mar 2022 07:50:43 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx>
wrote:
On Fri, 2022-03-04 at 14:28 +0200, Jarkko Sakkinen wrote:
With SGX1 an enclave needs to be created with its maximum memory demands
allocated. Pages cannot be added to an enclave after it is initialized.
SGX2 introduces a new function, ENCLS[EAUG], that can be used to add
pages
to an initialized enclave. With SGX2 the enclave still needs to set
aside
address space for its maximum memory demands during enclave creation,
but
all pages need not be added before enclave initialization. Pages can be
added during enclave runtime.
Add support for dynamically adding pages to an initialized enclave with
SGX_IOC_ENCLAVE_AUGMENT_PAGES, which performs EAUG's to a given range of
pages. Do not enforce any particular permissions from kernel, like is
done
for the pages added during the pre-initialization phase, as enclave
controls the final permissions and content for these pages by issuing
either ENCLU[EACCEPT] (empty RW) or ENCLU[EACCEPTCOPY] (arbitrary data
and
permissions).
Explicit EAUG ioctl is a better choice than an implicit EAUG from a page
fault handler because it allows to have O(1) number of kernel-enclave
round
trips for EAUG-EACCEPT{COPY} process, instead of O(n), as it is in the
case
when a page fault handler EAUG single page at a time.
Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Cc: Nathaniel McCallum <nathaniel@xxxxxxxxxxx>
Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
---
Is contained in sgx2-v2.1 branch of
git://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-sgx.git
I created sgx2-v2.2 branch, which has #PF EAUG removed. I
also moved selftests to the tail in the patch sets so that
it is easier to update them reflecting these and future
changes. Having them intervened makes things just complicated.
I focus now to implement mmap() for Enarx with this, so no
kselftest update just yet.
Roughly the sequence in Enarx is:
1. Enclave traps on syscall (opcode).
2. Host jumps to shim expection handler.
3. Enclave copies the mmap() arguments to a buffer outside
the enclave.
4. Enclave exists back to the host.
5. Host performs EAUG to the mmap range.
6. Host performs mmap() to the mmap range, which succeeds
given that vm_max_prot_bits is RWX (i.e. disabled for
dynamic pages).
7. Host jumps back to the enclave and execution continues
there in the mmap handler.
8. mmap handler does a series of EACCEPTCOPY operations for
the range with given permissions and empty page as the
input data.
EACCEPTCOPY will require target pages with RW in PTE. So you would need to
make mprotect to change PTE permissions afterwards depending on your
target permissions.
Without knowing much your context, if your intent is to
EACCEPTCOPY(EPCM.RX/EPCM.R, EPCM.pending), then I don't see how the page
can be used later without making it RW again first, and copy real data
into it.
So these empty EACCEPTCOPYs may be better just EACCEPTs(EPCM.RW,
EPCM.pending). Then after copy real data into the pages, you do
EMODPE/EMODPR as needed.
EACCEPTCOPY would make more sense when you already have data to be copied
into the EAUG'd but pending EPC pages.
Some details might differ a bit but this gives the basic idea.
I like the fact the roud-trips are kind of in control and not
variable, and it is pretty easy to use to implement the basic
syscall behaviour. This has of course some corner cases but
my sequence describes the main flow anyway.
Take it or leave it but this does give at least for me a sound
way to implement my workload. I'll use this up until my changes
have been inducted to the original patch set, or it starts to
look sane with other solutions. The original patch set simply
does not work for us at all.
BR, Jarkko
