Fix out-of-bounds memory accesses with virtual_root=""

The CGit configuration variable virtual_root is normalized so that it
does not have a trailing '/' character, but it is allowed to be empty
(the empty string and NULL have different meanings here) and there is
code that is insufficiently cautious when checking if it ends in a '/':

	if (virtual_root[strlen(virtual_root) - 1] != '/')

Clearly this check is redundant, but rather than simply removing it we
get a slight efficiency improvement by switching the normalization so
that the virtual_root variable always ends in '/'.  Do this with a new
"ensure_end" helper.

Signed-off-by: John Keeping <john@keeping.me.uk>

cgit.c

@@ -155,9 +155,7 @@ static void config_cb(const char *name, const char *value)
 	else if (!strcmp(name, "strict-export"))
 		ctx.cfg.strict_export = xstrdup(value);
 	else if (!strcmp(name, "virtual-root")) {
-		ctx.cfg.virtual_root = trim_end(value, '/');
-		if (!ctx.cfg.virtual_root && (!strcmp(value, "/")))
-			ctx.cfg.virtual_root = "";
+		ctx.cfg.virtual_root = ensure_end(value, '/');
 	} else if (!strcmp(name, "nocache"))
 		ctx.cfg.nocache = atoi(value);
 	else if (!strcmp(name, "noplainemail"))
@@ -833,11 +831,8 @@ int main(int argc, const char **argv)
 	 * that virtual-root equals SCRIPT_NAME, minus any possibly
 	 * trailing slashes.
 	 */
-	if (!ctx.cfg.virtual_root && ctx.cfg.script_name) {
-		ctx.cfg.virtual_root = trim_end(ctx.cfg.script_name, '/');
-		if (!ctx.cfg.virtual_root)
-			ctx.cfg.virtual_root = "";
-	}
+	if (!ctx.cfg.virtual_root && ctx.cfg.script_name)
+		ctx.cfg.virtual_root = ensure_end(ctx.cfg.script_name, '/');
 
 	/* If no url parameter is specified on the querystring, lets
 	 * use PATH_INFO as url. This allows cgit to work with virtual

cgit.h

@@ -190,7 +190,7 @@ struct cgit_config {
 	char *script_name;
 	char *section;
 	char *repository_sort;
-	char *virtual_root;
+	char *virtual_root;	/* Always ends with '/'. */
 	char *strict_export;
 	int cache_size;
 	int cache_dynamic_ttl;
@@ -300,6 +300,7 @@ extern int chk_positive(int result, char *msg);
 extern int chk_non_negative(int result, char *msg);
 
 extern char *trim_end(const char *str, char c);
+extern char *ensure_end(const char *str, char c);
 extern char *strlpart(char *txt, int maxlen);
 extern char *strrpart(char *txt, int maxlen);
 

shared.c

@@ -115,6 +115,21 @@ char *trim_end(const char *str, char c)
 	return xstrndup(str, len);
 }
 
+char *ensure_end(const char *str, char c)
+{
+	size_t len = strlen(str);
+	char *result;
+
+	if (len && str[len - 1] == c)
+		return xstrndup(str, len);
+
+	result = xmalloc(len + 2);
+	memcpy(result, str, len);
+	result[len] = '/';
+	result[len + 1] = '\0';
+	return result;
+}
+
 char *strlpart(char *txt, int maxlen)
 {
 	char *result;

ui-shared.c

@@ -57,7 +57,7 @@ const char *cgit_hosturl()
 const char *cgit_rooturl()
 {
 	if (ctx.cfg.virtual_root)
-		return fmt("%s/", ctx.cfg.virtual_root);
+		return ctx.cfg.virtual_root;
 	else
 		return ctx.cfg.script_name;
 }
@@ -65,7 +65,7 @@ const char *cgit_rooturl()
 char *cgit_repourl(const char *reponame)
 {
 	if (ctx.cfg.virtual_root) {
-		return fmt("%s/%s/", ctx.cfg.virtual_root, reponame);
+		return fmt("%s%s/", ctx.cfg.virtual_root, reponame);
 	} else {
 		return fmt("?r=%s", reponame);
 	}
@@ -78,7 +78,7 @@ char *cgit_fileurl(const char *reponame, const char *pagename,
 	char *delim;
 
 	if (ctx.cfg.virtual_root) {
-		tmp = fmt("%s/%s/%s/%s", ctx.cfg.virtual_root, reponame,
+		tmp = fmt("%s%s/%s/%s", ctx.cfg.virtual_root, reponame,
 			  pagename, (filename ? filename:""));
 		delim = "?";
 	} else {
@@ -126,11 +126,9 @@ static void site_url(const char *page, const char *search, const char *sort, int
 {
 	char *delim = "?";
 
-	if (ctx.cfg.virtual_root) {
+	if (ctx.cfg.virtual_root)
 		html_attr(ctx.cfg.virtual_root);
-		if (ctx.cfg.virtual_root[strlen(ctx.cfg.virtual_root) - 1] != '/')
-			html("/");
-	} else
+	else
 		html(ctx.cfg.script_name);
 
 	if (page) {
@@ -201,8 +199,6 @@ static char *repolink(const char *title, const char *class, const char *page,
 	html(" href='");
 	if (ctx.cfg.virtual_root) {
 		html_url_path(ctx.cfg.virtual_root);
-		if (ctx.cfg.virtual_root[strlen(ctx.cfg.virtual_root) - 1] != '/')
-			html("/");
 		html_url_path(ctx.repo->url);
 		if (ctx.repo->url[strlen(ctx.repo->url) - 1] != '/')
 			html("/");