x86/sgx: Improve comments for sgx_encl_lookup/alloc_backing()

Modify the comments for sgx_encl_lookup_backing() and for
sgx_encl_alloc_backing() to indicate that they take a reference
which must be dropped with a call to sgx_encl_put_backing().
Make sgx_encl_lookup_backing() static for now, and change the
name of sgx_encl_get_backing() to __sgx_encl_get_backing() to
make it more clear that sgx_encl_get_backing() is an internal
function.
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/all/YtUs3MKLzFg+rqEV@zn.tnic/

arch/x86/kernel/cpu/sgx/encl.c

@@ -12,6 +12,9 @@
 #include "encls.h"
 #include "sgx.h"
 
+static int sgx_encl_lookup_backing(struct sgx_encl *encl, unsigned long page_index,
+			    struct sgx_backing *backing);
+
 #define PCMDS_PER_PAGE (PAGE_SIZE / sizeof(struct sgx_pcmd))
 /*
  * 32 PCMD entries share a PCMD page. PCMD_FIRST_MASK is used to
@@ -914,7 +917,7 @@ static struct page *sgx_encl_get_backing_page(struct sgx_encl *encl,
 }
 
 /**
- * sgx_encl_get_backing() - Pin the backing storage
+ * __sgx_encl_get_backing() - Pin the backing storage
  * @encl:	an enclave pointer
  * @page_index:	enclave page index
  * @backing:	data for accessing backing storage for the page
@@ -926,7 +929,7 @@ static struct page *sgx_encl_get_backing_page(struct sgx_encl *encl,
  *   0 on success,
  *   -errno otherwise.
  */
-static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
+static int __sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
 			 struct sgx_backing *backing)
 {
 	pgoff_t page_pcmd_off = sgx_encl_get_backing_page_pcmd_offset(encl, page_index);
@@ -1001,7 +1004,7 @@ static struct mem_cgroup *sgx_encl_get_mem_cgroup(struct sgx_encl *encl)
 }
 
 /**
- * sgx_encl_alloc_backing() - allocate a new backing storage page
+ * sgx_encl_alloc_backing() - create a new backing storage page
  * @encl:	an enclave pointer
  * @page_index:	enclave page index
  * @backing:	data for accessing backing storage for the page
@@ -1009,7 +1012,9 @@ static struct mem_cgroup *sgx_encl_get_mem_cgroup(struct sgx_encl *encl)
  * When called from ksgxd, sets the active memcg from one of the
  * mms in the enclave's mm_list prior to any backing page allocation,
  * in order to ensure that shmem page allocations are charged to the
- * enclave.
+ * enclave.  Create a backing page for loading data back into an EPC page with
+ * ELDU.  This function takes a reference on a new backing page which
+ * must be dropped with a corresponding call to sgx_encl_put_backing().
  *
  * Return:
  *   0 on success,
@@ -1022,7 +1027,7 @@ int sgx_encl_alloc_backing(struct sgx_encl *encl, unsigned long page_index,
 	struct mem_cgroup *memcg = set_active_memcg(encl_memcg);
 	int ret;
 
-	ret = sgx_encl_get_backing(encl, page_index, backing);
+	ret = __sgx_encl_get_backing(encl, page_index, backing);
 
 	set_active_memcg(memcg);
 	mem_cgroup_put(encl_memcg);
@@ -1040,15 +1045,17 @@ int sgx_encl_alloc_backing(struct sgx_encl *encl, unsigned long page_index,
  * 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 which is not accounted for.
+ * This function takes a reference on an existing backing page which must be
+ * dropped with a corresponding call to sgx_encl_put_backing().
  *
  * Return:
  *   0 on success,
  *   -errno otherwise.
  */
-int sgx_encl_lookup_backing(struct sgx_encl *encl, unsigned long page_index,
+static 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);
+	return __sgx_encl_get_backing(encl, page_index, backing);
 }
 
 /**

arch/x86/kernel/cpu/sgx/encl.h

@@ -107,8 +107,6 @@ bool current_is_ksgxd(void);
 void sgx_encl_release(struct kref *ref);
 int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm);
 const cpumask_t *sgx_encl_cpumask(struct sgx_encl *encl);
-int sgx_encl_lookup_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);
 void sgx_encl_put_backing(struct sgx_backing *backing);