[PATCH v2] netfilter: nfnetlink_osf: fix null-ptr-deref in nf_osf_ttl

Kito Xu (veritas501)
2026-04-14T10:49:09Z

nf_osf_ttl() calls __in_dev_get_rcu(skb->dev) and passes the result
to in_dev_for_each_ifa_rcu() without checking for NULL. When the
receiving device has no IPv4 configuration (ip_ptr is NULL),
__in_dev_get_rcu() returns NULL and in_dev_for_each_ifa_rcu()
dereferences it unconditionally, causing a kernel crash.

This can happen when a packet arrives on a device that has had its
IPv4 configuration removed (e.g., MTU set below IPV4_MIN_MTU causing
inetdev_destroy) or on a device that was never assigned an IPv4
address, while an xt_osf or nft_osf rule with TTL_LESS mode is
active and the packet TTL exceeds the fingerprint TTL.

Add a NULL check for in_dev before using it. When in_dev is NULL,
return 0 (no match) since source-address locality cannot be
determined without IPv4 addresses on the device.

KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
RIP: 0010:nf_osf_match_one+0x204/0xa70
Call Trace:
<IRQ>
nf_osf_match+0x2f8/0x780
xt_osf_match_packet+0x11c/0x1f0
ipt_do_table+0x7fe/0x12b0
nf_hook_slow+0xac/0x1e0
ip_rcv+0x123/0x370
__netif_receive_skb_one_core+0x166/0x1b0
process_backlog+0x197/0x590
__napi_poll+0xa1/0x540
net_rx_action+0x401/0xd80
handle_softirqs+0x19f/0x610
</IRQ>

Fixes: a218dc82f0b5 ("netfilter: nft_osf: Add ttl option support")
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Kito Xu (veritas501) <hxzene@gmail.com>
---
net/netfilter/nfnetlink_osf.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/net/netfilter/nfnetlink_osf.c b/net/netfilter/nfnetlink_osf.c
index d64ce21c7b55..dd2cbbd449e7 100644
--- a/net/netfilter/nfnetlink_osf.c
+++ b/net/netfilter/nfnetlink_osf.c
@@ -36,6 +36,9 @@ static inline int nf_osf_ttl(const struct sk_buff *skb,
const struct in_ifaddr *ifa;
int ret = 0;

+ if (!in_dev)
+ return 0;
+
if (ttl_check == NF_OSF_TTL_TRUE)
return ip->ttl == f_ttl;
if (ttl_check == NF_OSF_TTL_NOCHECK)
--
2.43.0