netfilter: nfnetlink_osf: avoid OOB read

The opt_num field is controlled by user mode and is not currently
validated inside the kernel. An attacker can take advantage of this to
trigger an OOB read and potentially leak information.

BUG: KASAN: slab-out-of-bounds in nf_osf_match_one+0xbed/0xd10 net/netfilter/nfnetlink_osf.c:88
Read of size 2 at addr ffff88804bc64272 by task poc/6431

CPU: 1 PID: 6431 Comm: poc Not tainted 6.0.0-rc4 #1
Call Trace:
 nf_osf_match_one+0xbed/0xd10 net/netfilter/nfnetlink_osf.c:88
 nf_osf_find+0x186/0x2f0 net/netfilter/nfnetlink_osf.c:281
 nft_osf_eval+0x37f/0x590 net/netfilter/nft_osf.c:47
 expr_call_ops_eval net/netfilter/nf_tables_core.c:214
 nft_do_chain+0x2b0/0x1490 net/netfilter/nf_tables_core.c:264
 nft_do_chain_ipv4+0x17c/0x1f0 net/netfilter/nft_chain_filter.c:23
 [..]

Also add validation to genre, subtype and version fields.

Fixes: 11eeef41 ("netfilter: passive OS fingerprint xtables match")
Reported-by: Lucas Leong <wmliang@infosec.exchange>
Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nfnetlink_osf.c

@@ -315,6 +315,14 @@ static int nfnl_osf_add_callback(struct sk_buff *skb,
 
 	f = nla_data(osf_attrs[OSF_ATTR_FINGER]);
 
+	if (f->opt_num > ARRAY_SIZE(f->opt))
+		return -EINVAL;
+
+	if (!memchr(f->genre, 0, MAXGENRELEN) ||
+	    !memchr(f->subtype, 0, MAXGENRELEN) ||
+	    !memchr(f->version, 0, MAXGENRELEN))
+		return -EINVAL;
+
 	kf = kmalloc(sizeof(struct nf_osf_finger), GFP_KERNEL);
 	if (!kf)
 		return -ENOMEM;