post-process-pe: set EFI_IMAGE_DLLCHARACTERISTICS_NX_COMPAT

Currently, system firmware has no means to discover that an EFI
Application is compatible with the security feature variously known as
NX or w^x.

Since at least Revision 8.1, the PE spec supports setting a flag the
Optional Header's DllCharacteristics field to inform loaders that an
application supports being loaded with NX enabled.

In the case of UEFI, there are several things that should be enabled if
this flag is set:

- EFI_BOOT_SERVICES.AllocatePages() with MemoryType = EfiLoaderCode,
  EfiBootServicesCode, EfiRuntimeServicesCode, etc, currently must set
  memory as rwx.  This flag set implies that rw- is appropriate, and
  that the application knows how to use the EFI_MEMORY_ATTRIBUTE
  protocol to change that to r-x.
- EFI_BOOT_SERVICES.AllocatePool() - same as AllocatePages()
- EFI_BOOT_SERVICES.LoadImage()
  - currently must set the stack as rwx. This flag states that it is
    allowed to be rw-
  - currently a binary can probably have writable PLTs?  This flag
    allows the loader to not set them writable
- I have heard that some firmwares have the 0 page mapped rwx.
  Obviously this should not be done.

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

post-process-pe.c

@@ -330,6 +330,30 @@ load_pe(const char *const file, void *const data, const size_t datasize,
  errx(1, "%s: Security directory extends past end", file);
 }
 
+static void
+set_dll_characteristics(PE_COFF_LOADER_IMAGE_CONTEXT *ctx)
+{
+ uint16_t oldflags, newflags;
+
+ if (image_is_64_bit(ctx->PEHdr)) {
+ oldflags = ctx->PEHdr->Pe32Plus.OptionalHeader.DllCharacteristics;
+ } else {
+ oldflags = ctx->PEHdr->Pe32.OptionalHeader.DllCharacteristics;
+ }
+
+ newflags = oldflags | EFI_IMAGE_DLLCHARACTERISTICS_NX_COMPAT;
+ if (oldflags == newflags)
+ return;
+
+ debug(INFO, "Updating DLL Characteristics from 0x%04hx to 0x%04hx\n",
+ oldflags, newflags);
+ if (image_is_64_bit(ctx->PEHdr)) {
+ ctx->PEHdr->Pe32Plus.OptionalHeader.DllCharacteristics = newflags;
+ } else {
+ ctx->PEHdr->Pe32.OptionalHeader.DllCharacteristics = newflags;
+ }
+}
+
 static void
 fix_timestamp(PE_COFF_LOADER_IMAGE_CONTEXT *ctx)
 {
@@ -417,6 +441,8 @@ handle_one(char *f)
 
  load_pe(f, map, sz, &ctx);
 
+ set_dll_characteristics(&ctx);
+
  fix_timestamp(&ctx);
 
  fix_checksum(&ctx, map, sz);