[PATCH v2 1/2] MdeModulePkg/MdeModulePkg.dec/.uni: clarify PCDs usage

[Jian J Wang <jian.j.wang@intel.com>]

> v2 changes:
>    Newly added patch to clarify PCDs usage.

BZ#1116: https://bugzilla.tianocore.org/show_bug.cgi?id=1116

The usage of following PCDs described in MdeModulePkg.dec don't match
the implementation exactly. This patch updates related description in
both .dec and .uni files to avoid confusion in platform configuration.

  PcdSetNxForStack
  PcdImageProtectionPolicy
  PcdDxeNxMemoryProtectionPolicy

The main change is at the statement on how to handle the FALSE or 0
setting value in those PCDs. Current statement says the implementation
should unset or disable related features but in fact the related code
just do nothing (leave it to AS-IS). That means the result might be
disabled, or might be not. It depends on other features or platform
policy.

For example, if one don't want to enforce NX onto stack memory, he/she
needs to set PcdSetNxForStack to FALSE as well as to clear BIT4 of
PcdDxeNxMemoryProtectionPolicy.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
---
 MdeModulePkg/MdeModulePkg.dec | 20 +++++++++++++++-----
 MdeModulePkg/MdeModulePkg.uni | 13 +++++++++----
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/MdeModulePkg/MdeModulePkg.dec b/MdeModulePkg/MdeModulePkg.dec
index 74a699cbb7..6566b57fd4 100644
--- a/MdeModulePkg/MdeModulePkg.dec
+++ b/MdeModulePkg/MdeModulePkg.dec
@@ -1288,17 +1288,23 @@
   ## Set image protection policy. The policy is bitwise.
   #  If a bit is set, the image will be protected by DxeCore if it is aligned.
   #   The code section becomes read-only, and the data section becomes non-executable.
-  #  If a bit is clear, the image will not be protected.<BR><BR>
+  #  If a bit is clear, nothing will be done to image code/data sections.<BR><BR>
   #    BIT0       - Image from unknown device. <BR>
   #    BIT1       - Image from firmware volume.<BR>
+  #  <BR>
+  #  Note: If a bit is cleared, the data section could be still non-executable if
+  #  PcdDxeNxMemoryProtectionPolicy is enabled for EfiLoaderData, EfiBootServicesData
+  #  and/or EfiRuntimeServicesData.<BR>
+  #  <BR>
   # @Prompt Set image protection policy.
   # @ValidRange 0x80000002 | 0x00000000 - 0x0000001F
   gEfiMdeModulePkgTokenSpaceGuid.PcdImageProtectionPolicy|0x00000002|UINT32|0x00001047
 
   ## Set DXE memory protection policy. The policy is bitwise.
   #  If a bit is set, memory regions of the associated type will be mapped
-  #  non-executable.<BR><BR>
-  #
+  #  non-executable.<BR>
+  #  If a bit is cleared, nothing will be done to associated type of memory.<BR>
+  #  <BR>
   # Below is bit mask for this PCD: (Order is same as UEFI spec)<BR>
   #  EfiReservedMemoryType          0x0001<BR>
   #  EfiLoaderCode                  0x0002<BR>
@@ -1890,8 +1896,12 @@
   #  For the DxeIpl and the DxeCore are both X64, set NX for stack feature also require PcdDxeIplBuildPageTables be TRUE.<BR>
   #  For the DxeIpl and the DxeCore are both IA32 (PcdDxeIplSwitchToLongMode is FALSE), set NX for stack feature also require
   #  IA32 PAE is supported and Execute Disable Bit is available.<BR>
-  #   TRUE  - to set NX for stack.<BR>
-  #   FALSE - Not to set NX for stack.<BR>
+  #  <BR>
+  #  Note: If this PCD is set to FALSE, NX could be still applied to stack due to PcdDxeNxMemoryProtectionPolicy enabled for
+  #  EfiBootServicesData.<BR>
+  #  <BR>
+  #   TRUE  - Set NX for stack.<BR>
+  #   FALSE - Do nothing for stack.<BR>
   # @Prompt Set NX for stack.
   gEfiMdeModulePkgTokenSpaceGuid.PcdSetNxForStack|FALSE|BOOLEAN|0x0001006f
 
diff --git a/MdeModulePkg/MdeModulePkg.uni b/MdeModulePkg/MdeModulePkg.uni
index 080b8a62c0..61befdc1e4 100644
--- a/MdeModulePkg/MdeModulePkg.uni
+++ b/MdeModulePkg/MdeModulePkg.uni
@@ -345,8 +345,9 @@
                                                                                   "For the DxeIpl and the DxeCore are both X64, set NX for stack feature also require PcdDxeIplBuildPageTables be TRUE.<BR>"
                                                                                   "For the DxeIpl and the DxeCore are both IA32 (PcdDxeIplSwitchToLongMode is FALSE), set NX for stack feature also require"
                                                                                   "IA32 PAE is supported and Execute Disable Bit is available.<BR>"
-                                                                                  "TRUE  - to set NX for stack.<BR>"
-                                                                                  "FALSE - Not to set NX for stack.<BR>"
+                                                                                  "Note: If this PCD is set to FALSE, NX could be still applied to stack due to PcdDxeNxMemoryProtectionPolicy enabled for EfiBootServicesData.<BR>"
+                                                                                  "TRUE  - Set NX for stack.<BR>"
+                                                                                  "FALSE - Do nothing for stack.<BR>"
 
 #string STR_gEfiMdeModulePkgTokenSpaceGuid_PcdAcpiS3Enable_PROMPT  #language en-US "ACPI S3 Enable"
 
@@ -1098,15 +1099,19 @@
 #string STR_gEfiMdeModulePkgTokenSpaceGuid_PcdImageProtectionPolicy_HELP  #language en-US "Set image protection policy. The policy is bitwise.\n"
                                                                                           "If a bit is set, the image will be protected by DxeCore if it is aligned.\n"
                                                                                           "The code section becomes read-only, and the data section becomes non-executable.\n"
-                                                                                          "If a bit is clear, the image will not be protected.<BR><BR>\n"
+                                                                                          "If a bit is clear, nothing will be done to image code/data sections.<BR><BR>\n"
                                                                                           "BIT0       - Image from unknown device. <BR>\n"
                                                                                           "BIT1       - Image from firmware volume.<BR>"
+                                                                                          "Note: If a bit is cleared, the data section could be still non-executable if\n"
+                                                                                          "PcdDxeNxMemoryProtectionPolicy is enabled for EfiLoaderData, EfiBootServicesData\n"
+                                                                                          "and/or EfiRuntimeServicesData.<BR>"
 
 #string STR_gEfiMdeModulePkgTokenSpaceGuid_PcdDxeNxMemoryProtectionPolicy_PROMPT  #language en-US "Set DXE memory protection policy."
 
 #string STR_gEfiMdeModulePkgTokenSpaceGuid_PcdDxeNxMemoryProtectionPolicy_HELP  #language en-US "Set DXE memory protection policy. The policy is bitwise.\n"
                                                                                                 "If a bit is set, memory regions of the associated type will be mapped\n"
-                                                                                                "non-executable.<BR><BR>\n"
+                                                                                                "non-executable.<BR>\n"
+                                                                                                "If a bit is cleared, nothing will be done to associated type of memory.<BR><BR>\n"
                                                                                                 "\n"
                                                                                                 "Below is bit mask for this PCD: (Order is same as UEFI spec)<BR>\n"
                                                                                                 "EfiReservedMemoryType          0x0001<BR>\n"
-- 
2.16.2.windows.1