x86/boot: Fix early command-line parsing when matching at end

The x86 early command line parsing in cmdline_find_option_bool() is
buggy. If it matches a specified 'option' all the way to the end of the
command-line, it will consider it a match.

For instance,

  cmdline = "foo";
  cmdline_find_option_bool(cmdline, "fool");

will return 1. This is particularly annoying since we have actual FPU
options like "noxsave" and "noxsaves" So, command-line "foo bar noxsave"
will match *BOTH* a "noxsave" and "noxsaves". (This turns out not to be
an actual problem because "noxsave" implies "noxsaves", but it's still
confusing.)

To fix this, we simplify the code and stop tracking 'len'. 'len'
was trying to indicate either the NULL terminator *OR* the end of a
non-NULL-terminated command line at 'COMMAND_LINE_SIZE'. But, each of the
three states is *already* checking 'cmdline' for a NULL terminator.

We _only_ need to check if we have overrun 'COMMAND_LINE_SIZE', and that
we can do without keeping 'len' around.

Also add some commends to clarify what is going on.
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: fenghua.yu@intel.com
Cc: yu-cheng.yu@intel.com
Link: http://lkml.kernel.org/r/20151222225238.9AEB560C@viggo.jf.intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/lib/cmdline.c

@@ -21,12 +21,14 @@ static inline int myisspace(u8 c)
  * @option: option string to look for
  *
  * Returns the position of that @option (starts counting with 1)
- * or 0 on not found.
+ * or 0 on not found.  @option will only be found if it is found
+ * as an entire word in @cmdline.  For instance, if @option="car"
+ * then a cmdline which contains "cart" will not match.
  */
 int cmdline_find_option_bool(const char *cmdline, const char *option)
 {
 	char c;
-	int len, pos = 0, wstart = 0;
+	int pos = 0, wstart = 0;
 	const char *opptr = NULL;
 	enum {
 		st_wordstart = 0,	/* Start of word/after whitespace */
@@ -37,11 +39,14 @@ int cmdline_find_option_bool(const char *cmdline, const char *option)
 	if (!cmdline)
 		return -1;      /* No command line */
 
-	len = min_t(int, strlen(cmdline), COMMAND_LINE_SIZE);
-	if (!len)
+	if (!strlen(cmdline))
 		return 0;
 
-	while (len--) {
+	/*
+	 * This 'pos' check ensures we do not overrun
+	 * a non-NULL-terminated 'cmdline'
+	 */
+	while (pos < COMMAND_LINE_SIZE) {
 		c = *(char *)cmdline++;
 		pos++;
 
@@ -58,17 +63,26 @@ int cmdline_find_option_bool(const char *cmdline, const char *option)
 			/* fall through */
 
 		case st_wordcmp:
-			if (!*opptr)
+			if (!*opptr) {
+				/*
+				 * We matched all the way to the end of the
+				 * option we were looking for.  If the
+				 * command-line has a space _or_ ends, then
+				 * we matched!
+				 */
 				if (!c || myisspace(c))
 					return wstart;
 				else
 					state = st_wordskip;
-			else if (!c)
+			} else if (!c) {
+				/*
+				 * Hit the NULL terminator on the end of
+				 * cmdline.
+				 */
 				return 0;
-			else if (c != *opptr++)
+			} else if (c != *opptr++) {
 				state = st_wordskip;
-			else if (!len)		/* last word and is matching */
-				return wstart;
+			}
 			break;
 
 		case st_wordskip: