[{"id":3678954,"web_url":"http://patchwork.ozlabs.org/comment/3678954/","msgid":"","list_archive_url":null,"date":"2026-04-18T07:53:22","subject":"Re: [PATCH 2/2 nf] netfilter: nfnetlink_osf: fix potential NULL\n dereference in ttl check","submitter":{"id":1315,"url":"http://patchwork.ozlabs.org/api/people/1315/","name":"Pablo Neira Ayuso","email":"pablo@netfilter.org"},"content":"On Fri, Apr 17, 2026 at 06:20:57PM +0200, Fernando Fernandez Mancera wrote:\n> The nf_osf_ttl() function accessed skb->dev to perform a local interface\n> address lookup without verifying that the device pointer was valid.\n> \n> Additionally, the implementation utilized an in_dev_for_each_ifa_rcu\n> loop to match the packet source address against local interface\n> addresses. It assumed that packets from the same subnet should not see a\n> decrement on the initial TTL. A packet might appear it is from the same\n> subnet but it actually isn't especially in modern environments with\n> containers and virtual switching.\n> \n> Remove the device dereference and interface loop. Replace the logic with\n> a switch statement that evaluates the TTL according to the ttl_check.\n> \n> Fixes: 11eeef41d5f6 (\"netfilter: passive OS fingerprint xtables match\")\n> Reported-by: Kito Xu (veritas501) \n> Closes: https://lore.kernel.org/netfilter-devel/20260414074556.2512750-1-hxzene@gmail.com/\n> Signed-off-by: Fernando Fernandez Mancera \n\nReviewed-by: Pablo Neira Ayuso \n\n> ---\n> Note: if some help is needed during the backport I can assist.\n> ---\n> net/netfilter/nfnetlink_osf.c | 22 +++++++---------------\n> 1 file changed, 7 insertions(+), 15 deletions(-)\n> \n> diff --git a/net/netfilter/nfnetlink_osf.c b/net/netfilter/nfnetlink_osf.c\n> index f58267986453..f0d1e596e146 100644\n> --- a/net/netfilter/nfnetlink_osf.c\n> +++ b/net/netfilter/nfnetlink_osf.c\n> @@ -31,26 +31,18 @@ EXPORT_SYMBOL_GPL(nf_osf_fingers);\n> static inline int nf_osf_ttl(const struct sk_buff *skb,\n> \t\t\t int ttl_check, unsigned char f_ttl)\n> {\n> -\tstruct in_device *in_dev = __in_dev_get_rcu(skb->dev);\n> \tconst struct iphdr *ip = ip_hdr(skb);\n> -\tconst struct in_ifaddr *ifa;\n> -\tint ret = 0;\n> \n> -\tif (ttl_check == NF_OSF_TTL_TRUE)\n> +\tswitch (ttl_check) {\n> +\tcase NF_OSF_TTL_TRUE:\n> \t\treturn ip->ttl == f_ttl;\n> -\tif (ttl_check == NF_OSF_TTL_NOCHECK)\n> -\t\treturn 1;\n> -\telse if (ip->ttl <= f_ttl)\n> +\t\tbreak;\n> +\tcase NF_OSF_TTL_NOCHECK:\n> \t\treturn 1;\n> -\n> -\tin_dev_for_each_ifa_rcu(ifa, in_dev) {\n> -\t\tif (inet_ifa_match(ip->saddr, ifa)) {\n> -\t\t\tret = (ip->ttl == f_ttl);\n> -\t\t\tbreak;\n> -\t\t}\n> +\tcase NF_OSF_TTL_LESS:\n> +\tdefault:\n> +\t\treturn ip->ttl <= f_ttl;\n> \t}\n> -\n> -\treturn ret;\n> }\n> \n> struct nf_osf_hdr_ctx {\n> -- \n> 2.53.0\n>","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","netfilter-devel@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=netfilter.org header.i=@netfilter.org\n header.a=rsa-sha256 header.s=2025 header.b=ATjZEqTT;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c04:e001:36c::12fc:5321; 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