[PATCH V3 0/5] SGX shmem backing store issue

Reinette Chatre
2022-05-12T21:51:56Z

V2:
https://lore.kernel.org/linux-sgx/cover.1652131695.git.reinette.chatre@intel.com/

Changes since V2:
- Expand audience of series to include stable team, x86 maintainers, and lkml.
- Mark pages as dirty after receiving important data, not before. (Dave)
- Improve changelogs.
- Add Haitao and Jarkko's tags.
- Fix incorrect exit if address does not have enclave page associated. (Dave)
- See individual patches for detailed changes.

First version of series was submitted with incomplete fixes as RFC V1:
https://lore.kernel.org/linux-sgx/cover.1651171455.git.reinette.chatre@intel.com/

Changes since RFC V1:
- Remaining issue was root-caused with debugging help from Dave.
Patch 4/5 is new and eliminates all occurences of the ENCLS[ELDU] related
WARN.
- Drop "x86/sgx: Do not free backing memory on ENCLS[ELDU] failure" from
series. ENCLS[ELDU] failure is not recoverable so it serves no purpose to
keep the backing memory that could not be restored to the enclave.
- Patch 1/5 is new and refactors sgx_encl_put_backing() to only put
references to pages and not also mark the pages as dirty. (Dave)
- Mark PCMD page dirty before (not after) writing data to it. (Dave)
- Patch 5/5 is new and adds debug code to WARN if PCMD page is ever
found to contain data after it is truncated. (Dave)

== Cover Letter ==

Hi Everybody,

Haitao reported encountering the following WARN while stress testing SGX
with the SGX2 series [1] applied:

ELDU returned 1073741837 (0x4000000d)
WARNING: CPU: 72 PID: 24407 at arch/x86/kernel/cpu/sgx/encl.c:81 sgx_encl_eldu+0x3cf/0x400
...
Call Trace:
<TASK>
? xa_load+0x6e/0xa0
__sgx_encl_load_page+0x3d/0x80
sgx_encl_load_page_in_vma+0x4a/0x60
sgx_vma_fault+0x7f/0x3b0
? sysvec_call_function_single+0x66/0xd0
? asm_sysvec_call_function_single+0x12/0x20
__do_fault+0x39/0x110
__handle_mm_fault+0x1222/0x15a0
handle_mm_fault+0xdb/0x2c0
do_user_addr_fault+0x1d1/0x650
? exit_to_user_mode_prepare+0x45/0x170
exc_page_fault+0x76/0x170
? asm_exc_page_fault+0x8/0x30
asm_exc_page_fault+0x1e/0x30
...
</TASK>

ENCLS[ELDU] is returning a #GP when attempting to load an enclave
page from the backing store into the enclave.

Haitao's stress testing involves running two concurrent loops of the SGX2
selftests on a system with 4GB EPC memory. One of the tests is modified
to reduce the oversubscription heap size:

diff --git a/tools/testing/selftests/sgx/main.c b/tools/testing/selftests/sgx/main.c
index d480c2dd2858..12008789325b 100644
--- a/tools/testing/selftests/sgx/main.c
+++ b/tools/testing/selftests/sgx/main.c
@@ -398,7 +398,7 @@ TEST_F_TIMEOUT(enclave, unclobbered_vdso_oversubscribed_remove, 900)
* Create enclave with additional heap that is as big as all
* available physical SGX memory.
*/
- total_mem = get_total_epc_mem();
+ total_mem = get_total_epc_mem()/16;
ASSERT_NE(total_mem, 0);
TH_LOG("Creating an enclave with %lu bytes heap may take a while ...",
total_mem);

If the the test compiled with above snippet is renamed as "test_sgx_small"
and the original renamed as "test_sgx_large" the two concurrent loops are
run as follows:

(for i in $(seq 1 999); do echo "Iteration $i"; sudo ./test_sgx_large; done ) > log.large 2>&1
(for i in $(seq 1 9999); do echo "Iteration $i"; sudo ./test_sgx_small; done ) > log.small 2>&1

If the SGX driver is modified to always WARN when ENCLS[ELDU] encounters a #GP
(see below) then the WARN appears after a few iterations of "test_sgx_large"
and shows up throughout the testing.

diff --git a/arch/x86/kernel/cpu/sgx/encls.h b/arch/x86/kernel/cpu/sgx/encls.h
index 99004b02e2ed..68c1dbc84ed3 100644
--- a/arch/x86/kernel/cpu/sgx/encls.h
+++ b/arch/x86/kernel/cpu/sgx/encls.h
@@ -18,7 +18,7 @@
#define ENCLS_WARN(r, name) { \
do { \
int _r = (r); \
- WARN_ONCE(_r, "%s returned %d (0x%x)\n", (name), _r, _r); \
+ WARN(_r, "%s returned %d (0x%x)\n", (name), _r, _r); \
} while (0); \
}

I learned the following during investigation of the issue:
* Reverting commit 08999b2489b4 ("x86/sgx: Free backing memory after
faulting the enclave page") resolves the issue. With that commit
reverted the concurrent selftest loops can run to completion without
encountering any WARNs.

* The issue is also resolved if only the calls (introduced by commit
08999b2489b4 ("x86/sgx: Free backing memory after faulting the enclave
page") ) to sgx_encl_truncate_backing_page() within __sgx_encl_eldu()
are disabled.

* ENCLS[ELDU] faults with #GP when the provided PCMD data is all zeroes. It
does so because of a check that is not documented in the SDM. In this
check a page type of zero indicates an SECS page is being loaded into the
enclave, but since the SECS data is not empty the instruction faults with
#GP.

The fixes in this series address scenarios where the PCMD data in the
backing store may not be correct. While the SGX2 tests uncovered these
issues, the fixes are not related to SGX2 and apply on top of v5.18-rc6.
There are no occurences of the WARN when the stress testing is performed
with this series applied.

Thank you very much

Reinette

[1] https://lore.kernel.org/lkml/cover.1649878359.git.reinette.chatre@intel.com/

Reinette Chatre (5):
x86/sgx: Disconnect backing page references from dirty status
x86/sgx: Mark PCMD page as dirty when modifying contents
x86/sgx: Obtain backing storage page with enclave mutex held
x86/sgx: Fix race between reclaimer and page fault handler
x86/sgx: Ensure no data in PCMD page after truncate

arch/x86/kernel/cpu/sgx/encl.c | 113 ++++++++++++++++++++++++++++++---
arch/x86/kernel/cpu/sgx/encl.h | 2 +-
arch/x86/kernel/cpu/sgx/main.c | 13 ++--
3 files changed, 114 insertions(+), 14 deletions(-)

base-commit: c5eb0a61238dd6faf37f58c9ce61c9980aaffd7a
--
2.25.1