[efi] Distribute available entropy within stack cookie

Several of the values used to compute a stack cookie (in the absence
of a viable entropy source) will tend to have either all-zeroes or
all-ones in the higher order bits.  Rotate the values in order to
distribute the (minimal) available entropy more evenly.

Suggested-by: Pete Beck <pete.beck@ioactive.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/interface/efi/efi_init.c b/src/interface/efi/efi_init.c
index df46bb17bf37f4c67e454d89c96692a2abd7dec0..284c39b358f5da47b45b542bae9695a2a3b1d5df 100644 (file)
--- a/src/interface/efi/efi_init.c
+++ b/src/interface/efi/efi_init.c
@@ -22,6 +22,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <string.h>
 #include <errno.h>
 #include <ipxe/init.h>
+#include <ipxe/rotate.h>
 #include <ipxe/efi/efi.h>
 #include <ipxe/efi/efi_driver.h>
 #include <ipxe/efi/Protocol/LoadedImage.h>
@@ -105,19 +106,29 @@ static void * efi_find_table ( EFI_GUID *guid ) {
  */
 __attribute__ (( noinline )) unsigned long
 efi_stack_cookie ( EFI_HANDLE handle ) {
+       unsigned long cookie = 0;
+       unsigned int rotation = ( 8 * sizeof ( cookie ) / 4 );
 
        /* 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
+        */
+       cookie ^= ( ( unsigned long ) handle );
+       cookie = roll ( cookie, rotation );
+       cookie ^= ( ( unsigned long ) &handle );
+       cookie = roll ( cookie, rotation );
+       cookie ^= profile_timestamp();
+       cookie = roll ( cookie, rotation );
+       cookie ^= build_id;
+
+       /* 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.
+        * lower-order bits.
         */
-       return ( ( ( ( unsigned long ) handle ) ^
-                  ( ( unsigned long ) &handle ) ^
-                  profile_timestamp() ^ build_id ) << 8 );
+       cookie <<= 8;
+
+       return cookie;
 }
 
 /**