[RFC PATCH v2.1 05/30] Documentation/x86: Document SGX permission details

[Jarkko Sakkinen <jarkko@kernel.org>]

From: Reinette Chatre <reinette.chatre@intel.com>

Provide summary of the various permissions involved in
managing access to enclave pages. This summary documents
the foundation for additions related to runtime managing of
enclave page permissions that is made possible with SGX2.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
 Documentation/x86/sgx.rst | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/Documentation/x86/sgx.rst b/Documentation/x86/sgx.rst
index 265568a9292c..89ff924b1480 100644
--- a/Documentation/x86/sgx.rst
+++ b/Documentation/x86/sgx.rst
@@ -71,16 +71,34 @@ The processor tracks EPC pages in a hardware metadata structure called the
 which describes the owning enclave, access rights and page type among the other
 things.
 
-EPCM permissions are separate from the normal page tables.  This prevents the
-kernel from, for instance, allowing writes to data which an enclave wishes to
-remain read-only.  EPCM permissions may only impose additional restrictions on
-top of normal x86 page permissions.
-
 For all intents and purposes, the SGX architecture allows the processor to
 invalidate all EPCM entries at will.  This requires that software be prepared to
 handle an EPCM fault at any time.  In practice, this can happen on events like
 power transitions when the ephemeral key that encrypts enclave memory is lost.
 
+Details about enclave page permissions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+EPCM permissions are separate from the normal page tables.  This prevents the
+kernel from, for instance, allowing writes to data which an enclave wishes
+to remain read-only.
+
+Three permission masks are relevant to SGX:
+
+* EPCM permissions.
+* Page Table Entry (PTE) permissions.
+* Virtual Memory Area (VMA) permissions.
+
+An enclave is only able to access an enclave page if all three permission
+masks enable it to do so.
+
+The relationships between the different permission masks are:
+
+* An SGX VMA can only be created if its permissions are the same or weaker
+  than the EPCM permissions.
+* PTEs are installed to match the EPCM permissions, but not be more
+  relaxed than the VMA permissions.
+
 Application interface
 =====================
 
-- 
2.35.1