[PATCH] 9p: Make uname and remotename parsing more robust

[Markus Armbruster <armbru@redhat.com>]

match_strcpy() is a somewhat creepy function: the caller needs to make
sure that the destination buffer is big enough, and when he screws up
or forgets, match_strcpy() happily overruns the buffer.

There's exactly one customer: v9fs_parse_options().  I believe it
currently can't overflow its buffer, but that's not exactly obvious.

The source string is a substring of the mount options.  The kernel
silently truncates those to PAGE_SIZE bytes, including the terminating
zero.  See compat_sys_mount() and do_mount().

The destination buffer is obtained from __getname(), which allocates
from name_cachep, which is initialized by vfs_caches_init() for size
PATH_MAX.

We're safe as long as PATH_MAX <= PAGE_SIZE.  PATH_MAX is 4096.  As
far as I know, the smallest PAGE_SIZE is also 4096.

Here's a patch that makes the code a bit more obviously correct.  It
doesn't depend on PATH_MAX <= PAGE_SIZE.

Signed-off-by: Markus Armbruster <armbru@redhat.com>


diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index fbb12da..e42948b 100644
--- a/fs/9p/v9fs.c
+++ b/fs/9p/v9fs.c
@@ -135,10 +135,10 @@ static void v9fs_parse_options(struct v9fs_session_info *v9ses)
 			v9ses->trans = v9fs_match_trans(&args[0]);
 			break;
 		case Opt_uname:
-			match_strcpy(v9ses->uname, &args[0]);
+			match_strlcpy(v9ses->uname, &args[0], PATH_MAX);
 			break;
 		case Opt_remotename:
-			match_strcpy(v9ses->aname, &args[0]);
+			match_strlcpy(v9ses->aname, &args[0], PATH_MAX);
 			break;
 		case Opt_legacy:
 			v9ses->flags &= ~V9FS_EXTENDED;
diff --git a/include/linux/parser.h b/include/linux/parser.h
index 26b2bdf..7dcd050 100644
--- a/include/linux/parser.h
+++ b/include/linux/parser.h
@@ -29,5 +29,5 @@ int match_token(char *, match_table_t table, substring_t args[]);
 int match_int(substring_t *, int *result);
 int match_octal(substring_t *, int *result);
 int match_hex(substring_t *, int *result);
-void match_strcpy(char *, const substring_t *);
+size_t match_strlcpy(char *, const substring_t *, size_t);
 char *match_strdup(const substring_t *);
diff --git a/lib/parser.c b/lib/parser.c
index 703c8c1..4f0cbc0 100644
--- a/lib/parser.c
+++ b/lib/parser.c
@@ -182,18 +182,25 @@ int match_hex(substring_t *s, int *result)
 }
 
 /**
- * match_strcpy: - copies the characters from a substring_t to a string
- * @to: string to copy characters to.
- * @s: &substring_t to copy
+ * match_strlcpy: - Copy the characters from a substring_t to a sized buffer
+ * @dest: where to copy to
+ * @src: &substring_t to copy
+ * @size: size of destination buffer
  *
- * Description: Copies the set of characters represented by the given
- * &substring_t @s to the c-style string @to. Caller guarantees that @to is
- * large enough to hold the characters of @s.
+ * Description: Copy the characters in &substring_t @src to the
+ * c-style string @dest.  Copy no more than @size - 1 characters, plus
+ * the terminating NUL.  Return length of @src.
  */
-void match_strcpy(char *to, const substring_t *s)
+size_t match_strlcpy(char *dest, const substring_t *src, size_t size)
 {
-	memcpy(to, s->from, s->to - s->from);
-	to[s->to - s->from] = '\0';
+	size_t ret = src->to - src->from;
+
+	if (size) {
+		size_t len = ret >= size ? size - 1 : ret;
+		memcpy(dest, src->from, len);
+		dest[len] = '\0';
+	}
+	return ret;
 }
 
 /**
@@ -206,9 +213,10 @@ void match_strcpy(char *to, const substring_t *s)
  */
 char *match_strdup(const substring_t *s)
 {
-	char *p = kmalloc(s->to - s->from + 1, GFP_KERNEL);
+	size_t sz = s->to - s->from + 1;
+	char *p = kmalloc(sz, GFP_KERNEL);
 	if (p)
-		match_strcpy(p, s);
+		match_strlcpy(p, s, sz);
 	return p;
 }
 
@@ -216,5 +224,5 @@ EXPORT_SYMBOL(match_token);
 EXPORT_SYMBOL(match_int);
 EXPORT_SYMBOL(match_octal);
 EXPORT_SYMBOL(match_hex);
-EXPORT_SYMBOL(match_strcpy);
+EXPORT_SYMBOL(match_strlcpy);
 EXPORT_SYMBOL(match_strdup);