netfilter: nf_ct_sip: correct allowed characters in Call-ID SIP header

Current parsing methods for SIP header Call-ID do not check correctly all
characters allowed by RFC 3261. In particular "," character is allowed
instead of "'" character. As a result Call-ID headers like the following
are discarded by IPVS SIP persistence engine.

Call-ID: -.!%*_+`'~()<>:\"/[]?{}

Above example is composed using all non-alphanumeric characters listed
in RFC 3261 for Call-ID header syntax.

Proposed fix is in nf_conntrack_sip module; function iswordc() checks this
range: (c >= '(' && c <= '/') which includes these characters: ()*+,-./
They are all allowed except ",". Instead "'" is not included in the list.

Below is an extract of relevant SIP ABNF syntax.

Call-ID  =  ( "Call-ID" / "i" ) HCOLON callid
callid   =  word [ "@" word ]

HCOLON  =  *( SP / HTAB ) ":" SWS
SWS     =  [LWS] ; sep whitespace
LWS     =  [*WSP CRLF] 1*WSP ; linear whitespace
WSP     =  SP / HTAB
word    =  1*(alphanum / "-" / "." / "!" / "%" / "*" /
           "_" / "+" / "`" / "'" / "~" /
           "(" / ")" / "<" / ">" /
           ":" / "\" / DQUOTE /
           "/" / "[" / "]" / "?" /
           "{" / "}" )

Signed-off-by: Marco Angaroni <marcoangaroni@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_sip.c

@@ -83,9 +83,10 @@ static int digits_len(const struct nf_conn *ct, const char *dptr,
 static int iswordc(const char c)
 {
 	if (isalnum(c) || c == '!' || c == '"' || c == '%' ||
-	    (c >= '(' && c <= '/') || c == ':' || c == '<' || c == '>' ||
+	    (c >= '(' && c <= '+') || c == ':' || c == '<' || c == '>' ||
 	    c == '?' || (c >= '[' && c <= ']') || c == '_' || c == '`' ||
-	    c == '{' || c == '}' || c == '~')
+	    c == '{' || c == '}' || c == '~' || (c >= '-' && c <= '/') ||
+	    c == '\'')
 		return 1;
 	return 0;
 }