x86/kaslr: Make command line handling safer

Handle the possibility that the command line is NULL.

Replace open-coded strlen with a function call.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200727230801.3468620-2-nivedita@alum.mit.edu

arch/x86/boot/compressed/kaslr.c

@@ -268,15 +268,19 @@ static void parse_gb_huge_pages(char *param, char *val)
 static void handle_mem_options(void)
 {
 	char *args = (char *)get_cmd_line_ptr();
-	size_t len = strlen((char *)args);
+	size_t len;
 	char *tmp_cmdline;
 	char *param, *val;
 	u64 mem_size;
 
+	if (!args)
+		return;
+
 	if (!strstr(args, "memmap=") && !strstr(args, "mem=") &&
 		!strstr(args, "hugepages"))
 		return;
 
+	len = strlen(args);
 	tmp_cmdline = malloc(len + 1);
 	if (!tmp_cmdline)
 		error("Failed to allocate space for tmp_cmdline");
@@ -399,8 +403,7 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
 {
 	unsigned long init_size = boot_params->hdr.init_size;
 	u64 initrd_start, initrd_size;
-	u64 cmd_line, cmd_line_size;
-	char *ptr;
+	unsigned long cmd_line, cmd_line_size;
 
 	/*
 	 * Avoid the region that is unsafe to overlap during
@@ -421,16 +424,15 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
 	/* No need to set mapping for initrd, it will be handled in VO. */
 
 	/* Avoid kernel command line. */
-	cmd_line  = (u64)boot_params->ext_cmd_line_ptr << 32;
-	cmd_line |= boot_params->hdr.cmd_line_ptr;
+	cmd_line = get_cmd_line_ptr();
 	/* Calculate size of cmd_line. */
-	ptr = (char *)(unsigned long)cmd_line;
-	for (cmd_line_size = 0; ptr[cmd_line_size++];)
-		;
-	mem_avoid[MEM_AVOID_CMDLINE].start = cmd_line;
-	mem_avoid[MEM_AVOID_CMDLINE].size = cmd_line_size;
-	add_identity_map(mem_avoid[MEM_AVOID_CMDLINE].start,
-			 mem_avoid[MEM_AVOID_CMDLINE].size);
+	if (cmd_line) {
+		cmd_line_size = strlen((char *)cmd_line) + 1;
+		mem_avoid[MEM_AVOID_CMDLINE].start = cmd_line;
+		mem_avoid[MEM_AVOID_CMDLINE].size = cmd_line_size;
+		add_identity_map(mem_avoid[MEM_AVOID_CMDLINE].start,
+				 mem_avoid[MEM_AVOID_CMDLINE].size);
+	}
 
 	/* Avoid boot parameters. */
 	mem_avoid[MEM_AVOID_BOOTPARAMS].start = (unsigned long)boot_params;