[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>

1 files changed, 32 insertions, 0 deletions

diff --git a/src/include/ipxe/efi/efi.h b/src/include/ipxe/efi/efi.h
index 669e5364a..b8777ef47 100644
--- a/src/include/ipxe/efi/efi.h
+++ b/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 );