Enable the NX compatibility flag by default.

Currently by default, when we build shim we do not set the PE
NX-compatibility DLL Characteristic flag.  This signifies to the
firmware that shim (including the components it loads) is not prepared
for several related firmware changes:

- non-executable stack
- non-executable pages from AllocatePages()/AllocatePool()/etc.
- non-writable 0 page (not strictly related but some firmware will be
  transitioning at the same time)
- the need to use the UEFI 2.10 Memory Attribute Protocol to set page
  permissions.

This patch changes that default to be enabled by default.  Distributors
of shim will need to ensure that either their builds disable this bit
(using "post-process-pe -N"), or that the bootloaders and kernels you
support loading are all compliant with this change.  A new make
variable, POST_PROCESS_PE_FLAGS, has been added to simplify doing so.

Signed-off-by: Peter Jones <pjones@redhat.com>

BUILDING

@@ -78,6 +78,9 @@ Variables you could set to customize the build:
 - OSLABEL
  This is the label that will be put in BOOT$(EFI_ARCH).CSV for your OS.
  By default this is the same value as EFIDIR .
+- POST_PROCESS_PE_FLAGS
+ This allows you to add flags to the invocation of "post-process-pe", for
+ example to disable the NX compatibility flag.
 
 Vendor SBAT data:
 It will sometimes be requested by reviewers that a build includes extra

Make.defaults

@@ -139,6 +139,8 @@ CFLAGS = $(FEATUREFLAGS) \
  $(INCLUDES) \
  $(DEFINES)
 
+POST_PROCESS_PE_FLAGS =
+
 ifneq ($(origin OVERRIDE_SECURITY_POLICY), undefined)
  DEFINES += -DOVERRIDE_SECURITY_POLICY
 endif

Makefile

@@ -255,7 +255,7 @@ endif
  -j .rela* -j .dyn -j .reloc -j .eh_frame \
  -j .vendor_cert -j .sbat -j .sbatlevel \
  $(FORMAT) $< $@
- ./post-process-pe -vv $@
+ ./post-process-pe -vv $(POST_PROCESS_PE_FLAGS) $@
 
 ifneq ($(origin ENABLE_SHIM_HASH),undefined)
 %.hash : %.efi

post-process-pe.c

@@ -42,7 +42,7 @@ static int verbosity;
  0; \
  })
 
-static bool set_nx_compat = false;
+static bool set_nx_compat = true;
 
 typedef uint8_t UINT8;
 typedef uint16_t UINT16;