Re: [RFC PATCH 0/4] SGX shmem backing store issue

Reinette Chatre
2022-04-30T03:22:26Z

Hi Dave,

On 4/29/2022 12:45 PM, Dave Hansen wrote:
> On 4/29/22 11:50, Reinette Chatre wrote:
>> I am not familiar with this area - is the PFN expected to be consistent? Do
>> you perhaps have an idea why the PFN of the PCMD page may have changed? This
>> is running with this series applied so the ELDU flow would do a lookup of the
>> PCMD page and not attempt to allocate a new one.
>
> First of all, cool! This is really good progress!
>
> It's possible for the PCMD shmem page to be swapped out and swapped back
> in. The pfn would be likely to change in that case. But, that seems
> highly unlikely in that short of a period of time.
>
> I'd dump out two more things:
>
> First, dump out page_pcmd_off, just like you're doing with page_index in
> case page_pcmd_off itself is getting botched. I looked but couldn't
> find anything obvious.
>
> Second, dump out the pfn in sgx_encl_truncate_backing_page(). It's
> possible that something is getting overly-aggressive and zapping the
> PCMD page too early. That would be easy to explain with that PCMD
> locking issue you discovered. But, your patch should have fixed that issue.
>
> For debugging, could you double-check that the PCMD page *is* empty
> around sgx_encl_truncate_backing_page()? If there's a race here you can
> also enlarge the race window by adding an msleep() or a spin loop
> somewhere after the memchr_inv().
>
> You could also hold an extra reference on the PCMD page, maybe something
> like the attached patch. That will let you inspect the actual page
> after it is *actually* truncated. There should never be data in the
> page there.

Thank you so much for pointing me in the right direction. With this
information I believe that I found a race condition. I did create a
fix for it and it has been running for more than an hour now without
a WARN _and_ without the -ENOENT error introduced in patch 4/4.

Please find below the description of the race and the fix.

Scenario is that the reclaimer is evicting a range of pages that
are being faulted right away.

Consider three enclave pages (ENCLAVE_A, ENCLAVE_B, and ENCLAVE_C)
sharing a PCMD page (PCMD_ABC). ENCLAVE_A and ENCLAVE_B are in the
enclave memory and ENCLAVE_C is in backing store. PCMD_ABC contains
just one entry, that of ENCLAVE_C.

Scenario proceeds where ENCLAVE_A is being evicted from the enclave
while ENCLAVE_C is faulted in.

Scenario is with this patch series applied.

sgx_reclaim_pages() {

...

/*
* Reclaim ENCLAVE_A
*/
mutex_lock(&encl->lock);
/*
* Get a reference to ENCLAVE_A's
* shmem page where enclave page
* encrypted data will be stored
* as well as a reference to the
* enclave page's PCMD data page,
* PCMD_ABC.
*/
sgx_encl_alloc_backing(...);
mutex_unlock(&encl->lock);

/*
* Fault ENCLAVE_C
*/

sgx_vma_fault() {

mutex_lock(&encl->lock);
/*
* Get reference to
* ENCLAVE_C's shmem page
* as well as PCMD_ABC.
*/
sgx_encl_get_backing(...)
/*
* load page back into
* enclave via ELDU
*/
/*
* Release reference to
* ENCLAVE_C' shmem page and
* PCMD_ABC.
*/
sgx_encl_put_backing(...);
/*
* PCMD_ABC is found empty so
* it and ENCLAVE_C's shmem page
* are truncated.
*/
/* Truncate ENCLAVE_C backing page */
sgx_encl_truncate_backing_page();
/* Truncate PCMD_ABC */
sgx_encl_truncate_backing_page();

mutex_unlock(&encl->lock);

...
}
mutex_lock(&encl->lock);
/*
* Write encrypted contents of
* ENCLAVE_A to ENCLAVE_A shmem
* page and its PCMD data to
* PCMD_ABC.
*/

sgx_encl_put_backing(...)

/*
* Reference to PCMD_ABC is
* dropped and it is truncated.
* ENCLAVE_A's PCMD data is lost.
*/
mutex_unlock(&encl->lock);
}

What happens next depends on whether it is ENCLAVE_A being faulted
in or ENCLAVE_B (or ENCLAVE_C) being evicted.

If ENCLAVE_A is faulted then the error introduced in patch 4/4 would
be encountered since lookup of PCMD_ABC fails. Before patch 4/4 a new
PCMD page would be allocated and then ENCLS[ELDU] would WARN on the PCMD
data.
If ENCLAVE_B is evicted first then a new PCMD_ABC would be allocated but
later when ENCLAVE_A is faulted the ENCLS[ELDU] instruction would #GP
during its checks of the PCMD value and the WARN would be encountered.

Below is the fix I am currently testing. It would not truncate the memory
if there is a reference to the PCMD page.

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 22ed886dc825..81c7bfc12b9b 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -98,11 +98,10 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
kunmap_atomic(pcmd_page);
kunmap_atomic((void *)(unsigned long)pginfo.contents);

- sgx_encl_put_backing(&b, true);
-
+ put_page(b.contents);
sgx_encl_truncate_backing_page(encl, page_index);

- if (pcmd_page_empty) {
+ if (pcmd_page_empty && && put_page_testzero(b.pcmd)) {
ida_free(&encl->pcmd_in_backing, PFN_DOWN(page_pcmd_off));
sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
}

I plan on letting the test keep running to make sure it really fixes the
issue. If all goes well patch 4/4 can be dropped and the fix included instead.

Thanks again for your guidance. It was tremendously helpful.

Reinette