x86/sgx: Free backing memory after faulting the enclave page

There is a limited amount of SGX memory (EPC) on each system. When that
memory is used up, SGX has its own swapping mechanism which is similar
in concept but totally separate from the core mm/* code. Instead of
swapping to disk, SGX swaps from EPC to normal RAM. That normal RAM
comes from a shared memory pseudo-file and can itself be swapped by the
core mm code. There is a hierarchy like this:

EPC <-> shmem <-> disk

After data is swapped back in from shmem to EPC, the shmem backing
storage needs to be freed. Currently, the backing shmem is not freed.
This effectively wastes the shmem while the enclave is running. The
memory is recovered when the enclave is destroyed and the backing
storage freed.

Sort this out by freeing memory with shmem_truncate_range(), as soon as
a page is faulted back to the EPC. In addition, free the memory for
PCMD pages as soon as all PCMD's in a page have been marked as unused
by zeroing its contents.

Cc: [e-mail]
Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Reported-by: Dave Hansen <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

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

···
 #include "sgx.h"
 
 /*
+ * Calculate byte offset of a PCMD struct associated with an enclave page. PCMD's
+ * follow right after the EPC data in the backing storage. In addition to the
+ * visible enclave pages, there's one extra page slot for SECS, before PCMD
+ * structs.
+ */
+static inline pgoff_t sgx_encl_get_backing_page_pcmd_offset(struct sgx_encl *encl,
+							    unsigned long page_index)
+{
+	pgoff_t epc_end_off = encl->size + sizeof(struct sgx_secs);
+
+	return epc_end_off + page_index * sizeof(struct sgx_pcmd);
+}
+
+/*
+ * Free a page from the backing storage in the given page index.
+ */
+static inline void sgx_encl_truncate_backing_page(struct sgx_encl *encl, unsigned long page_index)
+{
+	struct inode *inode = file_inode(encl->backing);
+
+	shmem_truncate_range(inode, PFN_PHYS(page_index), PFN_PHYS(page_index) + PAGE_SIZE - 1);
+}
+
+/*
  * ELDU: Load an EPC page as unblocked. For more info, see "OS Management of EPC
  * Pages" in the SDM.
  */
···
 {
 	unsigned long va_offset = encl_page->desc & SGX_ENCL_PAGE_VA_OFFSET_MASK;
 	struct sgx_encl *encl = encl_page->encl;
+	pgoff_t page_index, page_pcmd_off;
 	struct sgx_pageinfo pginfo;
 	struct sgx_backing b;
-	pgoff_t page_index;
+	bool pcmd_page_empty;
+	u8 *pcmd_page;
 	int ret;
 
 	if (secs_page)
···
 	else
 		page_index = PFN_DOWN(encl->size);
 
+	page_pcmd_off = sgx_encl_get_backing_page_pcmd_offset(encl, page_index);
+
 	ret = sgx_encl_get_backing(encl, page_index, &b);
 	if (ret)
 		return ret;
 
 	pginfo.addr = encl_page->desc & PAGE_MASK;
 	pginfo.contents = (unsigned long)kmap_atomic(b.contents);
-	pginfo.metadata = (unsigned long)kmap_atomic(b.pcmd) +
-			  b.pcmd_offset;
+	pcmd_page = kmap_atomic(b.pcmd);
+	pginfo.metadata = (unsigned long)pcmd_page + b.pcmd_offset;
 
 	if (secs_page)
 		pginfo.secs = (u64)sgx_get_epc_virt_addr(secs_page);
···
 		ret = -EFAULT;
 	}
 
-	kunmap_atomic((void *)(unsigned long)(pginfo.metadata - b.pcmd_offset));
+	memset(pcmd_page + b.pcmd_offset, 0, sizeof(struct sgx_pcmd));
+
+	/*
+	 * The area for the PCMD in the page was zeroed above.  Check if the
+	 * whole page is now empty meaning that all PCMD's have been zeroed:
+	 */
+	pcmd_page_empty = !memchr_inv(pcmd_page, 0, PAGE_SIZE);
+
+	kunmap_atomic(pcmd_page);
 	kunmap_atomic((void *)(unsigned long)pginfo.contents);
 
 	sgx_encl_put_backing(&b, false);
+
+	sgx_encl_truncate_backing_page(encl, page_index);
+
+	if (pcmd_page_empty)
+		sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
 
 	return ret;
 }
···
 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);
+	pgoff_t page_pcmd_off = sgx_encl_get_backing_page_pcmd_offset(encl, page_index);
 	struct page *contents;
 	struct page *pcmd;
 
···
 	if (IS_ERR(contents))
 		return PTR_ERR(contents);
 
-	pcmd = sgx_encl_get_backing_page(encl, pcmd_index);
+	pcmd = sgx_encl_get_backing_page(encl, PFN_DOWN(page_pcmd_off));
 	if (IS_ERR(pcmd)) {
 		put_page(contents);
 		return PTR_ERR(pcmd);
···
 	backing->page_index = page_index;
 	backing->contents = contents;
 	backing->pcmd = pcmd;
-	backing->pcmd_offset =
-		(page_index & (PAGE_SIZE / sizeof(struct sgx_pcmd) - 1)) *
-		sizeof(struct sgx_pcmd);
+	backing->pcmd_offset = page_pcmd_off & (PAGE_SIZE - 1);
 
 	return 0;
 }