[efi] Enable stack protection where possible

Enable -fstack-protector for EFI builds, where binary size is less
critical than for BIOS builds.

The stack cookie must be constructed immediately on entry, which
prohibits the use of any viable entropy source.  Construct a cookie by
XORing together various mildly random quantities to produce a value
that will at least not be identical on each run.

On detecting a stack corruption, attempt to call Exit() with an
appropriate error.  If that fails, then lock up the machine since
there is no other safe action that can be taken.

The old conditional check for support of -fno-stack-protector is
omitted since this flag dates back to GCC 4.1.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.efi

@@ -1,5 +1,13 @@
 # -*- makefile -*- : Force emacs to use Makefile mode
 
+# Enable stack protection if available
+#
+SPG_TEST = $(CC) -fstack-protector-strong -mstack-protector-guard=global \
+		 -x c -c /dev/null -o /dev/null >/dev/null 2>&1
+SPG_FLAGS := $(shell $(SPG_TEST) && $(ECHO) '-fstack-protector-strong ' \
+					    '-mstack-protector-guard=global')
+CFLAGS += $(SPG_FLAGS)
+
 # The EFI linker script
 #
 LDSCRIPT	= scripts/efi.lds

src/Makefile.housekeeping

@@ -146,17 +146,6 @@ define NEWLINE
 
 endef
 
-# Some widespread patched versions of gcc include -fstack-protector by
-# default, even when -ffreestanding is specified.  We therefore need
-# to disable -fstack-protector if the compiler supports it.
-#
-ifeq ($(CCTYPE),gcc)
-SP_TEST = $(CC) -fno-stack-protector -x c -c /dev/null \
-		-o /dev/null >/dev/null 2>&1
-SP_FLAGS := $(shell $(SP_TEST) && $(ECHO) '-fno-stack-protector')
-WORKAROUND_CFLAGS += $(SP_FLAGS)
-endif
-
 # gcc 4.4 generates .eh_frame sections by default, which distort the
 # output of "size".  Inhibit this.
 #
@@ -415,6 +404,13 @@ ifdef BIN
 incdirs :
 	@$(ECHO) $(INCDIRS)
 
+# Inhibit -fstack-protector (which is implicitly enabled in some
+# patched gcc versions) unless explicitly mentioned in CFLAGS.
+#
+ifeq ($(findstring -fstack-protector,$(CFLAGS)),)
+CFLAGS		+= -fno-stack-protector
+endif
+
 # Common flags
 #
 CFLAGS		+= $(foreach INC,$(INCDIRS),-I$(INC))

src/include/ipxe/efi/efi.h

@@ -65,6 +65,8 @@ typedef struct {} *EFI_HANDLE;
 
 #include <ipxe/tables.h>
 #include <ipxe/uuid.h>
+#include <ipxe/version.h>
+#include <ipxe/profile.h>
 
 /** An EFI protocol used by iPXE */
 struct efi_protocol {
@@ -272,6 +274,36 @@ extern void dbg_efi_protocols ( EFI_HANDLE handle );
 #define DBGCP_EFI_PROTOCOLS( ... )				\
 	DBGC_EFI_PROTOCOLS_IF ( PROFILE, ##__VA_ARGS__ )
 
+extern unsigned long __stack_chk_guard;
+extern unsigned long efi_stack_cookie ( EFI_HANDLE handle );
+extern void __stack_chk_fail ( void );
+
+/**
+ * Initialise stack cookie
+ *
+ * @v handle		Image handle
+ */
+static inline __attribute__ (( always_inline )) void
+efi_init_stack_guard ( EFI_HANDLE handle ) {
+
+	/* The calling function must not itself use stack protection,
+	 * since the change in the stack guard value would trigger a
+	 * false positive.
+	 *
+	 * There is unfortunately no way to annotate a function to
+	 * exclude the use of stack protection.  We must therefore
+	 * rely on correctly anticipating the compiler's decision on
+	 * the use of stack protection.
+	 *
+	 * The calculation of the stack cookie value deliberately
+	 * takes the address of a stack variable (to provide an
+	 * additional source of entropy).  This operation would
+	 * trigger the application of stack protection to the calling
+	 * function, and so must be externalised.
+	 */
+	__stack_chk_guard = efi_stack_cookie ( handle );
+}
+
 extern EFI_STATUS efi_init ( EFI_HANDLE image_handle,
 			     EFI_SYSTEM_TABLE *systab );
 

src/interface/efi/efi_init.c

@@ -47,6 +47,16 @@ int efi_shutdown_in_progress;
 /** Event used to signal shutdown */
 static EFI_EVENT efi_shutdown_event;
 
+/** Stack cookie */
+unsigned long __stack_chk_guard;
+
+/** Exit function
+ *
+ * Cached to minimise external dependencies when a stack check
+ * failure is triggered.
+ */
+static EFI_EXIT efi_exit;
+
 /* Forward declarations */
 static EFI_STATUS EFIAPI efi_unload ( EFI_HANDLE image_handle );
 
@@ -87,6 +97,29 @@ static void * efi_find_table ( EFI_GUID *guid ) {
 	return NULL;
 }
 
+/**
+ * Construct a stack cookie value
+ *
+ * @v handle		Image handle
+ * @ret cookie		Stack cookie
+ */
+__attribute__ (( noinline )) unsigned long
+efi_stack_cookie ( EFI_HANDLE handle ) {
+
+	/* There is no viable source of entropy available at this
+	 * point.  Construct a value that is at least likely to vary
+	 * between platforms and invocations.
+	 *
+	 * Ensure that the value contains a NUL byte, to act as a
+	 * runaway string terminator.  Construct the NUL using a shift
+	 * rather than a mask, to avoid losing valuable entropy in the
+	 * low-order bits.
+	 */
+	return ( ( ( ( unsigned long ) handle ) ^
+		   ( ( unsigned long ) &handle ) ^
+		   profile_timestamp() ^ build_id ) << 8 );
+}
+
 /**
  * Initialise EFI environment
  *
@@ -130,6 +163,9 @@ EFI_STATUS efi_init ( EFI_HANDLE image_handle,
 	DBGC ( systab, "EFI handle %p systab %p\n", image_handle, systab );
 	bs = systab->BootServices;
 
+	/* Store abort function pointer */
+	efi_exit = bs->Exit;
+
 	/* Look up used protocols */
 	for_each_table_entry ( prot, EFI_PROTOCOLS ) {
 		if ( ( efirc = bs->LocateProtocol ( &prot->guid, NULL,
@@ -240,3 +276,29 @@ static EFI_STATUS EFIAPI efi_unload ( EFI_HANDLE image_handle __unused ) {
 
 	return 0;
 }
+
+/**
+ * Abort on stack check failure
+ *
+ */
+__attribute__ (( noreturn )) void __stack_chk_fail ( void ) {
+	EFI_STATUS efirc;
+	int rc;
+
+	/* Report failure (when debugging) */
+	DBGC ( efi_systab, "EFI stack check failed (cookie %#lx); aborting\n",
+	       __stack_chk_guard );
+
+	/* Attempt to exit cleanly with an error status */
+	if ( efi_exit ) {
+		efirc = efi_exit ( efi_image_handle, EFI_COMPROMISED_DATA,
+				   0, NULL );
+		rc = -EEFI ( efirc );
+		DBGC ( efi_systab, "EFI stack check exit failed: %s\n",
+		       strerror ( rc ) );
+	}
+
+	/* If the exit fails for any reason, lock the system */
+	while ( 1 ) {}
+
+}

src/interface/efi/efidrvprefix.c

@@ -36,6 +36,9 @@ EFI_STATUS EFIAPI _efidrv_start ( EFI_HANDLE image_handle,
 				  EFI_SYSTEM_TABLE *systab ) {
 	EFI_STATUS efirc;
 
+	/* Initialise stack cookie */
+	efi_init_stack_guard ( image_handle );
+
 	/* Initialise EFI environment */
 	if ( ( efirc = efi_init ( image_handle, systab ) ) != 0 )
 		return efirc;

src/interface/efi/efiprefix.c

@@ -41,6 +41,9 @@ EFI_STATUS EFIAPI _efi_start ( EFI_HANDLE image_handle,
 	EFI_STATUS efirc;
 	int rc;
 
+	/* Initialise stack cookie */
+	efi_init_stack_guard ( image_handle );
+
 	/* Initialise EFI environment */
 	if ( ( efirc = efi_init ( image_handle, systab ) ) != 0 )
 		goto err_init;