[PATCH 4/5] x86: fix early command-line parsing, when partial word match

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

cmdline_find_option_bool() keeps track of position in two strings:
1. the command-line
2. the option we are searchign for in the command-line

We plow through each character in the command-line one at a time,
always moving forward.  We move forward in the option ('opptr')
when we match characters in 'cmdline'.  We reset the 'opptr' only
when we go in to the 'st_wordstart' state.

But, if we fail to match an option because we see a space (
state=st_wordcmp, *opptr='\0',c=' '), we set state='st_wordskip'
and 'break', moving to the next character.  But, that move to
the next character is the one *after* the ' '.  This means that
we will miss a 'st_wordstart' state.

For instance, if we have

	cmdline = "foo fool";

and are searching for "fool", we have:

	"fool"
opptr = ----^

	"foo fool"
c = --------^

We see that 'l' != ' ', set state=st_wordskip, break, and then
move 'c', so:

	"foo fool"
c = ---------^

and are still in state=st_wordskip.  We will stay in wordskip
until we have skipped "fool", thus missing the option we were
looking for.  This *only* happens when you have a partially-
matching word followed by a matching one.

To fix this, we always fall *into* the 'st_wordskip' state when
we set it.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: fenghua.yu@xxxxxxxxx
Cc: yu-cheng.yu@xxxxxxxxx
---

 b/arch/x86/lib/cmdline.c |   18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff -puN arch/x86/lib/cmdline.c~x86-mid-option-match-command-line-parsing arch/x86/lib/cmdline.c
--- a/arch/x86/lib/cmdline.c~x86-mid-option-match-command-line-parsing	2015-12-17 13:54:53.804985689 -0800
+++ b/arch/x86/lib/cmdline.c	2015-12-17 13:54:53.808985871 -0800
@@ -74,18 +74,26 @@ static int __cmdline_find_option_bool(co
 				 */
 				if (!c || myisspace(c))
 					return wstart;
-				else
-					state = st_wordskip;
+				/*
+				 * We hit the end of the option, but _not_
+				 * the end of a word on the cmdline.  Not
+				 * a match.
+				 */
 			} else if (!c) {
 				/*
 				 * Hit the NULL terminator on the end of
 				 * cmdline.
 				 */
 				return 0;
-			} else if (c != *opptr++) {
-				state = st_wordskip;
+			} else if (c == *opptr++) {
+				/*
+				 * We are currently matching, so continue
+				 * to the next character on the cmdline.
+				 */
+				break;
 			}
-			break;
+			state = st_wordskip;
+			/* fall through */
 
 		case st_wordskip:
 			if (!c)
_
--
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