[PATCH V2 5/5] x86/sgx: Ensure no data in PCMD page after truncate

[Reinette Chatre <reinette.chatre@intel.com>]

A PCMD (Paging Crypto MetaData) page contains the PCMD
structures of enclave pages that have been encrypted and
moved to the shmem backing store. When all enclave pages
sharing a PCMD page are loaded in the enclave, there is no
need for the PCMD page and it can be truncated from the
backing store.

A few issues appeared around the truncation of PCMD pages. The
known issues have been addressed but the PCMD handling code could
be made more robust by loudly complaining if any new issue appears
in this area.

Add a check that will complain once with a WARN if the PCMD page is not
actually empty after it has been truncated. There should never be data
in the PCMD page at this point since it is always updated with the
enclave mutex held.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
 arch/x86/kernel/cpu/sgx/encl.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index d1d4e8572702..af972dbad965 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -183,12 +183,19 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
 	kunmap_atomic(pcmd_page);
 	kunmap_atomic((void *)(unsigned long)pginfo.contents);
 
+	get_page(b.pcmd);
 	sgx_encl_put_backing(&b);
 
 	sgx_encl_truncate_backing_page(encl, page_index);
 
-	if (pcmd_page_empty && !pcmd_page_in_use(encl, pcmd_first_page))
+	if (pcmd_page_empty && !pcmd_page_in_use(encl, pcmd_first_page)) {
 		sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
+		pcmd_page = kmap_atomic(b.pcmd);
+		WARN_ON_ONCE(memchr_inv(pcmd_page, 0, PAGE_SIZE));
+		kunmap_atomic(pcmd_page);
+	}
+
+	put_page(b.pcmd);
 
 	return ret;
 }
-- 
2.25.1