On Thu, May 12, 2022 at 02:50:59PM -0700, Reinette Chatre wrote:
Haitao reported encountering a WARN triggered by the ENCLS[ELDU] instruction faulting with a #GP.
The WARN is encountered when the reclaimer evicts a range of pages from the enclave when the same pages are faulted back right away.
The SGX backing storage is accessed on two paths: when there are insufficient free pages in the EPC the reclaimer works to move enclave pages to the backing storage and as enclaves access pages that have been moved to the backing storage they are retrieved from there as part of page fault handling.
An oversubscribed SGX system will often run the reclaimer and page fault handler concurrently and needs to ensure that the backing store is accessed safely between the reclaimer and the page fault handler. This is not the case because the reclaimer accesses the backing store without the enclave mutex while the page fault handler accesses the backing store with the enclave mutex.
Consider the scenario where a page is faulted while a page sharing a PCMD page with the faulted page is being reclaimed. The consequence is a race between the reclaimer and page fault handler, the reclaimer attempting to access a PCMD at the same time it is truncated by the page fault handler. This could result in lost PCMD data. Data may still be lost if the reclaimer wins the race, this is addressed in the following patch.
The reclaimer accesses pages from the backing storage without holding the enclave mutex and runs the risk of concurrently accessing the backing storage with the page fault handler that does access the backing storage with the enclave mutex held.
In the scenario below a PCMD page is truncated from the backing store after all its pages have been loaded in to the enclave at the same time the PCMD page is loaded from the backing store when one of its pages are reclaimed:
sgx_reclaim_pages() { sgx_vma_fault() { ... mutex_lock(&encl->lock); ... __sgx_encl_eldu() { ... if (pcmd_page_empty) { /*
- EPC page being reclaimed /*
- shares a PCMD page with an * PCMD page truncated
- enclave page that is being * while requested from
- faulted in. * reclaimer.
*/ */ sgx_encl_get_backing() <----------> sgx_encl_truncate_backing_page() } mutex_unlock(&encl->lock); } }
In this scenario there is a race between the reclaimer and the page fault handler when the reclaimer attempts to get access to the same PCMD page that is being truncated. This could result in the reclaimer writing to the PCMD page that is then truncated, causing the PCMD data to be lost, or in a new PCMD page being allocated. The lost PCMD data may still occur after protecting the backing store access with the mutex - this is fixed in the next patch. By ensuring the backing store is accessed with the mutex held the enclave page state can be made accurate with the SGX_ENCL_PAGE_BEING_RECLAIMED flag accurately reflecting that a page is in the process of being reclaimed.
Consistently protect the reclaimer's backing store access with the enclave's mutex to ensure that it can safely run concurrently with the page fault handler.
Cc: stable@vger.kernel.org Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer") Reported-by: Haitao Huang haitao.huang@intel.com Reviewed-by: Jarkko Sakkinen jarkko@kernel.org Tested-by: Jarkko Sakkinen jarkko@kernel.org Tested-by: Haitao Huang haitao.huang@intel.com Signed-off-by: Reinette Chatre reinette.chatre@intel.com
Changes since V2:
- Move description of "scenario a" to the new patch in series that marks page as dirty with enclave mutex held.
- Add Haitao's "Tested-by" tag.
- Add Jarkko's "Reviewed-by" and "Tested-by" tags.
arch/x86/kernel/cpu/sgx/main.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c index e71df40a4f38..ab4ec54bbdd9 100644 --- a/arch/x86/kernel/cpu/sgx/main.c +++ b/arch/x86/kernel/cpu/sgx/main.c @@ -310,6 +310,7 @@ static void sgx_reclaimer_write(struct sgx_epc_page *epc_page, sgx_encl_ewb(epc_page, backing); encl_page->epc_page = NULL; encl->secs_child_cnt--;
- sgx_encl_put_backing(backing);
if (!encl->secs_child_cnt && test_bit(SGX_ENCL_INITIALIZED, &encl->flags)) { ret = sgx_encl_get_backing(encl, PFN_DOWN(encl->size), @@ -381,11 +382,14 @@ static void sgx_reclaim_pages(void) goto skip; page_index = PFN_DOWN(encl_page->desc - encl_page->encl->base);
mutex_lock(&encl_page->encl->lock);
if (ret)
if (ret) {mutex_unlock(&encl_page->encl->lock); goto skip;}
mutex_lock(&encl_page->encl->lock);@@ -413,7 +417,6 @@ static void sgx_reclaim_pages(void) encl_page = epc_page->owner; sgx_reclaimer_write(epc_page, &backing[i]);
sgx_encl_put_backing(&backing[i]);kref_put(&encl_page->encl->refcount, sgx_encl_release); epc_page->flags &= ~SGX_EPC_PAGE_RECLAIMER_TRACKED; -- 2.25.1
Reviewed-by: Jarkko Sakkinen jarkko@kernel.org
BR, Jarkko