[PATCH v3 2/2] x86/sgx: account backing pages

[Kristen Carlson Accardi]

SGX may allow EPC pages to be overcommitted. If the system is
out of enclave memory, EPC pages are swapped to normal RAM via
a per enclave shared memory area. This shared memory is not
charged to the enclave or the task mapping it, making it hard
to account for using normal methods.

In order to avoid unlimited usage of normal RAM, enclaves must be
charged for each new page used for backing storage, and uncharged
when they are no longer using a backing page.

Modify the existing flow for requesting backing pages to reduce the
available backing page counter and confirm that the limit has not
been exceeded. Backing page usage for loading EPC pages back out of
the shared memory do not incur a charge.

When a backing page is released from usage, increment the available
backing page counter.

When swapping EPC pages to RAM, in addition to storing the page
contents, SGX must store some additional metadata to protect
against a malicious kernel when the page is swapped back in. This
additional metadata is called Paging Crypto MetaData. PCMD is
allocated from the same shared memory area as the backing page
contents and consumes RAM the same way.

PCMD is 128 bytes in size, and there is one PCMD structure per
page written to shared RAM. The page index for the PCMD page is
calculated from the page index of the backing page, so it is possible
that the PCMD structures are not packed into the minimum number of
pages possible. If 32 PCMDs can fit onto a single page, then PCMD
usage is 1/32 of total EPC pages. In the worst case, PCMD can
consume the same amount of RAM as EPC backing pages (1:1). For
simplicity, this implementation does not account for PCMD page usage.

Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
Tested-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/sgx/encl.c | 76 ++++++++++++++++++++++++++++++++--
 arch/x86/kernel/cpu/sgx/encl.h |  6 ++-
 arch/x86/kernel/cpu/sgx/main.c |  6 +--
 3 files changed, 80 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 001808e3901c..8be6f0592bdc 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -32,7 +32,7 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
 	else
 		page_index = PFN_DOWN(encl->size);
 
-	ret = sgx_encl_get_backing(encl, page_index, &b);
+	ret = sgx_encl_lookup_backing(encl, page_index, &b);
 	if (ret)
 		return ret;
 
@@ -407,6 +407,12 @@ void sgx_encl_release(struct kref *ref)
 			sgx_encl_free_epc_page(entry->epc_page);
 			encl->secs_child_cnt--;
 			entry->epc_page = NULL;
+		} else {
+			/*
+			 * If there is no epc_page, it means it has been
+			 * swapped out. Uncharge the backing storage.
+			 */
+			sgx_uncharge_mem();
 		}
 
 		kfree(entry);
@@ -574,8 +580,8 @@ static struct page *sgx_encl_get_backing_page(struct sgx_encl *encl,
  *   0 on success,
  *   -errno otherwise.
  */
-int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
-			 struct sgx_backing *backing)
+static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
+				struct sgx_backing *backing)
 {
 	pgoff_t pcmd_index = PFN_DOWN(encl->size) + 1 + (page_index >> 5);
 	struct page *contents;
@@ -601,6 +607,62 @@ int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
 	return 0;
 }
 
+/**
+ * sgx_encl_alloc_backing() - allocate a new backing storage page
+ * @encl:	an enclave pointer
+ * @page_index:	enclave page index
+ * @backing:	data for accessing backing storage for the page
+ *
+ * Confirm that the global overcommit limit has not been reached before
+ * requesting a new backing storage page for storing the encrypted contents
+ * and Paging Crypto MetaData (PCMD) of an enclave page. This is called when
+ * there is no existing backing page, just before writing to the backing
+ * storage with EWB.
+ *
+ * Return:
+ *   0 on success,
+ *   -errno otherwise.
+ */
+int sgx_encl_alloc_backing(struct sgx_encl *encl, unsigned long page_index,
+			   struct sgx_backing *backing)
+{
+	int ret;
+
+	if (sgx_charge_mem())
+		return -ENOMEM;
+
+	ret = sgx_encl_get_backing(encl, page_index, backing);
+	if (ret)
+		sgx_uncharge_mem();
+
+	return ret;
+}
+
+/**
+ * sgx_encl_lookup_backing() - retrieve an existing backing storage page
+ * @encl:	an enclave pointer
+ * @page_index:	enclave page index
+ * @backing:	data for accessing backing storage for the page
+ *
+ * Retrieve a backing page for loading data back into an EPC page with ELDU.
+ * This call does not cause a charge to the overcommit limit because a page
+ * has already been allocated, but has been swapped out or is in RAM
+ *
+ * It is the caller's responsibility to ensure that it is appropriate to
+ * use sgx_encl_lookup_backing() rather than sgx_encl_alloc_backing(). If
+ * lookup is not used correctly, this will cause an allocation that is
+ * not accounted for.
+ *
+ * Return:
+ *   0 on success,
+ *   -errno otherwise.
+ */
+int sgx_encl_lookup_backing(struct sgx_encl *encl, unsigned long page_index,
+			    struct sgx_backing *backing)
+{
+	return sgx_encl_get_backing(encl, page_index, backing);
+}
+
 /**
  * sgx_encl_put_backing() - Unpin the backing storage
  * @backing:	data for accessing backing storage for the page
@@ -608,9 +670,17 @@ int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
  */
 void sgx_encl_put_backing(struct sgx_backing *backing, bool do_write)
 {
+	/*
+	 * If the page is being written to by the reclaimer, it is
+	 * still in use and the backing page usage should not be
+	 * uncharged. However, if the page is not being written to,
+	 * it is no longer in use and may be uncharged.
+	 */
 	if (do_write) {
 		set_page_dirty(backing->pcmd);
 		set_page_dirty(backing->contents);
+	} else {
+		sgx_uncharge_mem();
 	}
 
 	put_page(backing->pcmd);
diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h
index fec43ca65065..8ffb8a83263f 100644
--- a/arch/x86/kernel/cpu/sgx/encl.h
+++ b/arch/x86/kernel/cpu/sgx/encl.h
@@ -105,8 +105,10 @@ int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
 
 void sgx_encl_release(struct kref *ref);
 int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm);
-int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
-			 struct sgx_backing *backing);
+int sgx_encl_alloc_backing(struct sgx_encl *encl, unsigned long page_index,
+			   struct sgx_backing *backing);
+int sgx_encl_lookup_backing(struct sgx_encl *encl, unsigned long page_index,
+			    struct sgx_backing *backing);
 void sgx_encl_put_backing(struct sgx_backing *backing, bool do_write);
 int sgx_encl_test_and_clear_young(struct mm_struct *mm,
 				  struct sgx_encl_page *page);
diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index 261e3702aef9..990f341bbd30 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -347,8 +347,8 @@ static void sgx_reclaimer_write(struct sgx_epc_page *epc_page,
 	encl->secs_child_cnt--;
 
 	if (!encl->secs_child_cnt && test_bit(SGX_ENCL_INITIALIZED, &encl->flags)) {
-		ret = sgx_encl_get_backing(encl, PFN_DOWN(encl->size),
-					   &secs_backing);
+		ret = sgx_encl_alloc_backing(encl, PFN_DOWN(encl->size),
+					     &secs_backing);
 		if (ret)
 			goto out;
 
@@ -418,7 +418,7 @@ static void sgx_reclaim_pages(void)
 			goto skip;
 
 		page_index = PFN_DOWN(encl_page->desc - encl_page->encl->base);
-		ret = sgx_encl_get_backing(encl_page->encl, page_index, &backing[i]);
+		ret = sgx_encl_alloc_backing(encl_page->encl, page_index, &backing[i]);
 		if (ret)
 			goto skip;
 
-- 
2.20.1