netfilter: x_tables: add and use xt_check_proc_name

recent and hashlimit both create /proc files, but only check that
name is 0 terminated.

This can trigger WARN() from procfs when name is "" or "/".
Add helper for this and then use it for both.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: <syzbot+0502b00edac2a0680b61@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter/x_tables.h

@@ -285,6 +285,8 @@ unsigned int *xt_alloc_entry_offsets(unsigned int size);
 bool xt_find_jump_offset(const unsigned int *offsets,
 			 unsigned int target, unsigned int size);
 
+int xt_check_proc_name(const char *name, unsigned int size);
+
 int xt_check_match(struct xt_mtchk_param *, unsigned int size, u_int8_t proto,
 		   bool inv_proto);
 int xt_check_target(struct xt_tgchk_param *, unsigned int size, u_int8_t proto,

net/netfilter/x_tables.c

@@ -423,6 +423,36 @@ textify_hooks(char *buf, size_t size, unsigned int mask, uint8_t nfproto)
 	return buf;
 }
 
+/**
+ * xt_check_proc_name - check that name is suitable for /proc file creation
+ *
+ * @name: file name candidate
+ * @size: length of buffer
+ *
+ * some x_tables modules wish to create a file in /proc.
+ * This function makes sure that the name is suitable for this
+ * purpose, it checks that name is NUL terminated and isn't a 'special'
+ * name, like "..".
+ *
+ * returns negative number on error or 0 if name is useable.
+ */
+int xt_check_proc_name(const char *name, unsigned int size)
+{
+	if (name[0] == '\0')
+		return -EINVAL;
+
+	if (strnlen(name, size) == size)
+		return -ENAMETOOLONG;
+
+	if (strcmp(name, ".") == 0 ||
+	    strcmp(name, "..") == 0 ||
+	    strchr(name, '/'))
+		return -EINVAL;
+
+	return 0;
+}
+EXPORT_SYMBOL(xt_check_proc_name);
+
 int xt_check_match(struct xt_mtchk_param *par,
 		   unsigned int size, u_int8_t proto, bool inv_proto)
 {

net/netfilter/xt_hashlimit.c

@@ -917,8 +917,9 @@ static int hashlimit_mt_check_v1(const struct xt_mtchk_param *par)
 	struct hashlimit_cfg3 cfg = {};
 	int ret;
 
-	if (info->name[sizeof(info->name) - 1] != '\0')
-		return -EINVAL;
+	ret = xt_check_proc_name(info->name, sizeof(info->name));
+	if (ret)
+		return ret;
 
 	ret = cfg_copy(&cfg, (void *)&info->cfg, 1);
 
@@ -935,8 +936,9 @@ static int hashlimit_mt_check_v2(const struct xt_mtchk_param *par)
 	struct hashlimit_cfg3 cfg = {};
 	int ret;
 
-	if (info->name[sizeof(info->name) - 1] != '\0')
-		return -EINVAL;
+	ret = xt_check_proc_name(info->name, sizeof(info->name));
+	if (ret)
+		return ret;
 
 	ret = cfg_copy(&cfg, (void *)&info->cfg, 2);
 
@@ -950,9 +952,11 @@ static int hashlimit_mt_check_v2(const struct xt_mtchk_param *par)
 static int hashlimit_mt_check(const struct xt_mtchk_param *par)
 {
 	struct xt_hashlimit_mtinfo3 *info = par->matchinfo;
+	int ret;
 
-	if (info->name[sizeof(info->name) - 1] != '\0')
-		return -EINVAL;
+	ret = xt_check_proc_name(info->name, sizeof(info->name));
+	if (ret)
+		return ret;
 
 	return hashlimit_mt_check_common(par, &info->hinfo, &info->cfg,
 					 info->name, 3);

net/netfilter/xt_recent.c

@@ -361,9 +361,9 @@ static int recent_mt_check(const struct xt_mtchk_param *par,
 				    info->hit_count, XT_RECENT_MAX_NSTAMPS - 1);
 		return -EINVAL;
 	}
-	if (info->name[0] == '\0' ||
-	    strnlen(info->name, XT_RECENT_NAME_LEN) == XT_RECENT_NAME_LEN)
-		return -EINVAL;
+	ret = xt_check_proc_name(info->name, sizeof(info->name));
+	if (ret)
+		return ret;
 
 	if (ip_pkt_list_tot && info->hit_count < ip_pkt_list_tot)
 		nstamp_mask = roundup_pow_of_two(ip_pkt_list_tot) - 1;