netfilter: x_tables: add and use xt_check_proc_name

commit b1d0a5d0cba4597c0394997b2d5fced3e3841b4e upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/netfilter/x_tables.h

@@ -247,6 +247,8 @@ 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

@@ -366,6 +366,36 @@ 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

@@ -668,8 +668,9 @@ static int hashlimit_mt_check(const struct xt_mtchk_param *par)
 
 	if (info->cfg.gc_interval == 0 || info->cfg.expire == 0)
 		return -EINVAL;
-	if (info->name[sizeof(info->name)-1] != '\0')
+	ret = xt_check_proc_name(info->name, sizeof(info->name));
-		return -EINVAL;
+	if (ret)
+		return ret;
 	if (par->family == NFPROTO_IPV4) {
 		if (info->cfg.srcmask > 32 || info->cfg.dstmask > 32)
 			return -EINVAL;

net/netfilter/xt_recent.c

@@ -364,9 +364,9 @@ 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' ||
+	ret = xt_check_proc_name(info->name, sizeof(info->name));
-	    strnlen(info->name, XT_RECENT_NAME_LEN) == XT_RECENT_NAME_LEN)
+	if (ret)
-		return -EINVAL;
+		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;