arm64: efi: Enable BTI codegen and add PE/COFF annotation

UEFI heavily relies on so-called protocols, which are essentially
tables populated with pointers to executable code, and these are invoked
indirectly using BR or BLR instructions.

This makes the EFI execution context vulnerable to attacks on forward
edge control flow, and so it would help if we could enable hardware
enforcement (BTI) on CPUs that implement it.

So let's no longer disable BTI codegen for the EFI stub, and set the
newly introduced PE/COFF header flag when the kernel is built with BTI
landing pads.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>

arch/arm64/kernel/efi-header.S

@@ -81,9 +81,47 @@
 	.quad	0					// CertificationTable
 	.quad	0					// BaseRelocationTable
 
-#ifdef CONFIG_DEBUG_EFI
+#if defined(CONFIG_DEBUG_EFI) || defined(CONFIG_ARM64_BTI_KERNEL)
 	.long	.Lefi_debug_table - .L_head		// DebugTable
 	.long	.Lefi_debug_table_size
+
+	/*
+	 * The debug table is referenced via its Relative Virtual Address (RVA),
+	 * which is only defined for those parts of the image that are covered
+	 * by a section declaration. Since this header is not covered by any
+	 * section, the debug table must be emitted elsewhere. So stick it in
+	 * the .init.rodata section instead.
+	 *
+	 * Note that the payloads themselves are permitted to have zero RVAs,
+	 * which means we can simply put those right after the section headers.
+	 */
+	__INITRODATA
+
+	.align	2
+.Lefi_debug_table:
+#ifdef CONFIG_DEBUG_EFI
+	// EFI_IMAGE_DEBUG_DIRECTORY_ENTRY
+	.long	0					// Characteristics
+	.long	0					// TimeDateStamp
+	.short	0					// MajorVersion
+	.short	0					// MinorVersion
+	.long	IMAGE_DEBUG_TYPE_CODEVIEW		// Type
+	.long	.Lefi_debug_entry_size			// SizeOfData
+	.long	0					// RVA
+	.long	.Lefi_debug_entry - .L_head		// FileOffset
+#endif
+#ifdef CONFIG_ARM64_BTI_KERNEL
+	.long	0					// Characteristics
+	.long	0					// TimeDateStamp
+	.short	0					// MajorVersion
+	.short	0					// MinorVersion
+	.long	IMAGE_DEBUG_TYPE_EX_DLLCHARACTERISTICS	// Type
+	.long	4					// SizeOfData
+	.long	0					// RVA
+	.long	.Lefi_dll_characteristics_ex - .L_head	// FileOffset
+#endif
+	.set	.Lefi_debug_table_size, . - .Lefi_debug_table
+	.previous
 #endif
 
 	// Section table
@@ -119,33 +157,6 @@
 	.set	.Lsection_count, (. - .Lsection_table) / 40
 
 #ifdef CONFIG_DEBUG_EFI
-	/*
-	 * The debug table is referenced via its Relative Virtual Address (RVA),
-	 * which is only defined for those parts of the image that are covered
-	 * by a section declaration. Since this header is not covered by any
-	 * section, the debug table must be emitted elsewhere. So stick it in
-	 * the .init.rodata section instead.
-	 *
-	 * Note that the EFI debug entry itself may legally have a zero RVA,
-	 * which means we can simply put it right after the section headers.
-	 */
-	__INITRODATA
-
-	.align	2
-.Lefi_debug_table:
-	// EFI_IMAGE_DEBUG_DIRECTORY_ENTRY
-	.long	0					// Characteristics
-	.long	0					// TimeDateStamp
-	.short	0					// MajorVersion
-	.short	0					// MinorVersion
-	.long	IMAGE_DEBUG_TYPE_CODEVIEW		// Type
-	.long	.Lefi_debug_entry_size			// SizeOfData
-	.long	0					// RVA
-	.long	.Lefi_debug_entry - .L_head		// FileOffset
-
-	.set	.Lefi_debug_table_size, . - .Lefi_debug_table
-	.previous
-
 .Lefi_debug_entry:
 	// EFI_IMAGE_DEBUG_CODEVIEW_NB10_ENTRY
 	.ascii	"NB10"					// Signature
@@ -157,6 +168,10 @@
 
 	.set	.Lefi_debug_entry_size, . - .Lefi_debug_entry
 #endif
+#ifdef CONFIG_ARM64_BTI_KERNEL
+.Lefi_dll_characteristics_ex:
+	.long	IMAGE_DLLCHARACTERISTICS_EX_FORWARD_CFI_COMPAT
+#endif
 
 	.balign	SEGMENT_ALIGN
 .Lefi_header_end:

drivers/firmware/efi/libstub/Makefile

@@ -23,8 +23,7 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ \
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)		+= -fpie $(DISABLE_STACKLEAK_PLUGIN) \
-				   -fno-unwind-tables -fno-asynchronous-unwind-tables \
-				   $(call cc-option,-mbranch-protection=none)
+				   -fno-unwind-tables -fno-asynchronous-unwind-tables
 cflags-$(CONFIG_ARM)		+= -DEFI_HAVE_STRLEN -DEFI_HAVE_STRNLEN \
 				   -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
 				   -DEFI_HAVE_STRCMP -fno-builtin -fpic \