x86/microcode/intel: Fix out of bounds memory access to the extended header

Improper pointer arithmetics when calculating the address of the
extended header could lead to an out of bounds memory read and kernel
panic.
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Link: http://lkml.kernel.org/r/20150225094125.GB30434@chrystal.uk.oracle.comSigned-off-by: Borislav Petkov <bp@suse.de>

arch/x86/kernel/cpu/microcode/intel_early.c

@@ -180,8 +180,7 @@ matching_model_microcode(struct microcode_header_intel *mc_header,
 	if (total_size <= data_size + MC_HEADER_SIZE)
 		return UCODE_NFOUND;
 
-	ext_header = (struct extended_sigtable *)
-		     mc_header + data_size + MC_HEADER_SIZE;
+	ext_header = (void *) mc_header + data_size + MC_HEADER_SIZE;
 	ext_sigcount = ext_header->count;
 	ext_sig = (void *)ext_header + EXT_HEADER_SIZE;
 
@@ -457,8 +456,7 @@ static void __ref show_saved_mc(void)
 		if (total_size <= data_size + MC_HEADER_SIZE)
 			continue;
 
-		ext_header = (struct extended_sigtable *)
-			     mc_saved_header + data_size + MC_HEADER_SIZE;
+		ext_header = (void *) mc_saved_header + data_size + MC_HEADER_SIZE;
 		ext_sigcount = ext_header->count;
 		ext_sig = (void *)ext_header + EXT_HEADER_SIZE;