kernel/git/stable/linux.git/net/ipv6, branch linux-rolling-lts

net: advertise TCP MSS from the configured MTU, not the learned PMTU

2026-09-11T09:49:42+00:00

commit 2640e64195948a601430d230c9864f5426574cde upstream. The MSS a host puts in its SYN tells the peer how big a segment it may send us. Right now we can shrink it with a PMTU we learned on our own send path, which is the wrong direction entirely. On asymmetric paths this bites - think DSR load balancers, where the request side goes through a smaller-MTU overlay. We learn a small PMTU going out, then advertise a small MSS, and the peer stays capped for the whole connection even though its path back to us is wide. MSS only shows up in the SYN and never grows back. On symmetric paths we lose nothing by dropping it either: the peer runs its own PMTU discovery and usually already knows the real path MTU. So work out the advertised MSS from the configured route or device MTU and ignore the learned PMTU. Our send side is unchanged, still clamped by tcp_current_mss(). Add ip_dst_mtu_configured()/ip6_dst_mtu_configured() and use them from the two default_advmss() paths. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Fixes: 164a5e7ad531 ("ipv4: ipv4_default_advmss() should use route mtu") Cc: stable@vger.kernel.org Signed-off-by: Jiayuan Chen Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260815070413.294559-1-jiayuan.chen@linux.dev Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

tcp: clamp route advmss to TCP_MIN_MSS

2026-09-11T09:49:42+00:00

[ Upstream commit 870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1 ] tcp_select_initial_window() assumes that callers never pass an MSS smaller than 1, but route-derived advmss values can violate that assumption. A too-small explicit RTAX_ADVMSS is one way to get there, but it is not the only one. The same divide-by-zero can also be reached through the "default advmss" path when RTAX_ADVMSS is left at 0 and the effective advmss is later driven down by route MTU and min_adv_mss. Introduce a tcp_dst_advmss() helper that clamps route advmss to TCP_MIN_MSS before TCP consumes it, and use it in the TCP paths that derive advmss from dst metrics. This keeps the effective MSS from dropping to zero before tcp_select_initial_window() rounds the receive window. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Reported-by: Vega Signed-off-by: Yong Wang Signed-off-by: Ren Wei Link: https://patch.msgid.link/251eaf8277fa7c66364c9815c5da01662d269181.1787074852.git.edragain@163.com Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

ipv4: use dst4_mtu() instead of dst_mtu()

2026-09-11T09:49:42+00:00

[ Upstream commit fe8570186f100b6cc499b2f7705946baf1388cde ] When we expect an IPv4 dst, use dst4_mtu() instead of dst_mtu() to save some code space. Signed-off-by: Eric Dumazet Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260130210303.3888261-8-edumazet@google.com Signed-off-by: Jakub Kicinski Stable-dep-of: 870a9e42ecc6 ("tcp: clamp route advmss to TCP_MIN_MSS") Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

ipv6: use dst6_mtu() instead of dst_mtu()

2026-09-11T09:49:41+00:00

[ Upstream commit b40f0130a23b6bab8b359e158a40af81f11f14be ] When we expect an IPv6 dst, use dst6_mtu() instead of dst_mtu() to save some code space. Due to current dst6_mtu() implementation, only convert users in IPv6 stack. Signed-off-by: Eric Dumazet Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260130210303.3888261-7-edumazet@google.com Signed-off-by: Jakub Kicinski Stable-dep-of: 870a9e42ecc6 ("tcp: clamp route advmss to TCP_MIN_MSS") Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

ipv6: add some unlikely()/likely() clauses in ip6_output.c

2026-09-11T09:49:41+00:00

[ Upstream commit 2855e4925416c5900a9c6cece11f4a2e56bf6e63 ] 1) daddr is unlikely a multicast in ip6_finish_output2(). 2) ip6_finish_output_gso_slowpath_drop() should not be called often. 3) ip6_fragment() should not be called often. 4) opt is unlikely to be set. 5) ip6_xmit() and ip6_forward() mostly sends not too big packets. 6) Most __ip6_make_skb() calls are for UDP packets, not ICMPV6 ones. Signed-off-by: Eric Dumazet Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260130210303.3888261-3-edumazet@google.com Signed-off-by: Jakub Kicinski Stable-dep-of: 870a9e42ecc6 ("tcp: clamp route advmss to TCP_MIN_MSS") Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

ipv6: pass proto by value to ipv6_push_nfrag_opts() and ipv6_push_frag_opts()

2026-09-11T09:49:41+00:00

[ Upstream commit 1bc46dd209667346c5aa56c351995e7c260696d0 ] With CONFIG_STACKPROTECTOR_STRONG=y, it is better to avoid passing a pointer to an automatic variable. Change these exported functions to return 'u8 proto' instead of void. - ipv6_push_nfrag_opts() - ipv6_push_frag_opts() For instance, replace ipv6_push_frag_opts(skb, opt, &proto); with: proto = ipv6_push_frag_opts(skb, opt, proto); Note that even after this change, ip6_xmit() has to use a stack canary because of @first_hop variable. Signed-off-by: Eric Dumazet Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260130210303.3888261-2-edumazet@google.com Signed-off-by: Jakub Kicinski Stable-dep-of: 870a9e42ecc6 ("tcp: clamp route advmss to TCP_MIN_MSS") Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

seg6: reset IP6CB after IPv6 decapsulation

2026-09-07T15:22:55+00:00

commit f967455fb2a5a2079b9eb5823e9ccf359174bf9f upstream. decap_and_validate() pulls the outer SRv6 headers and makes the inner packet the skb network header. The IPv6 control block still contains values collected while parsing the outer packet, including nhoff and extension-header flags. End.DX6 and End.DT6 route the inner IPv6 packet directly to the IPv6 input path. An unprivileged user can reach End.DT6 from a user and net namespace by installing a local SID and injecting an outer packet with Hop-by-Hop and Destination Options headers followed by an SRH and a minimal inner IPv6 packet. The outer extension headers leave a large nhoff in IP6CB. After decapsulation, ip6_protocol_deliver_rcu() uses that stale offset on the inner packet and reads beyond the skb head. KASAN reports: BUG: KASAN: slab-out-of-bounds in ip6_protocol_deliver_rcu ip6_protocol_deliver_rcu+0x1118/0x1450 ip6_input_finish+0x11b/0x240 seg6_local_input_core+0xed/0x2e0 lwtunnel_input+0x1e9/0x4e0 ipv6_rthdr_rcv+0x525f/0x6c50 ip6_protocol_deliver_rcu+0xcb7/0x1450 Before clearing IP6CB for an inner IPv6 packet, save its incoming interface index and L3 slave state. Restore both after the clear and set nhoff to the inner IPv6 base-header nexthdr field. Use IP6CB(skb)->iif rather than skb->skb_iif because VRF processing can replace skb_iif with the L3 master while IP6CB keeps the receiving interface. Preserve IP6SKB_L3SLAVE for the same reason. Fixes: d7a669dd2f8b ("ipv6: sr: add helper functions for seg6local") Cc: stable@vger.kernel.org Reported-by: Vega Signed-off-by: Zhiling Zou Reviewed-by: Andrea Mayer Signed-off-by: David S. Miller Signed-off-by: Greg Kroah-Hartman

net: cap advertised IP tunnel headroom

2026-09-07T15:22:55+00:00

commit 6b222adeb9340306e2ff97127c76117abb9b3df8 upstream. IP tunnel devices derive their advertised needed_headroom from lower output devices. A stack of user-created devices can make the derived value larger than the 16-bit skb header offsets can represent. Once IP output reserves it, skb head expansion can wrap those offsets. The runtime transmit path already caps a growing needed_headroom at 512. Apply the same cap when tunnel configuration publishes needed_headroom derived from a lower output device. Capping the advertised value is safe: IP tunnel transmit still expands the skb when a packet needs more headroom. A nonsensical stacked configuration can therefore incur an extra reallocation, but it cannot publish an unbounded reservation to upper layers. Fixes: 1a37e412a022 ("net: Use 16bits for *_headers fields of struct skbuff") Cc: stable@vger.kernel.org Reported-by: Vega Signed-off-by: Zhiling Zou Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/ba04a1fd6bfae2377607fad5d8f80f7eb80fd4c4.1786542637.git.zhilinz@nebusec.ai Signed-off-by: Paolo Abeni Signed-off-by: Greg Kroah-Hartman

ipv6: use RCU iterator to dump route exceptions

2026-09-07T15:22:39+00:00

commit 47cdab0d51aaa9bd85f8e4904585bd5bd4df4488 upstream. rt6_nh_dump_exceptions() uses hlist_for_each_entry() to iterate over RCU-protected exception lists. The caller holds rcu_read_lock(), but does not hold rt6_exception_lock, so rt6_insert_exception() can concurrently add an entry with hlist_add_head_rcu(). KCSAN reports this race (irrelevant details omitted): ================================================================== BUG: KCSAN: data-race in rt6_insert_exception / rt6_nh_dump_exceptions write (marked) to 0xffff8a7c44c59620 of 8 bytes by interrupt on cpu 5: rt6_insert_exception+0x3bb/0x760 __ip6_rt_update_pmtu+0x4fe/0x750 ip6_sk_update_pmtu+0x19a/0x3b0 udpv6_err+0x3ff/0x800 icmpv6_notify+0x1e1/0x440 icmpv6_rcv+0x8c0/0xab0 ip6_protocol_deliver_rcu+0x616/0x840 ip6_input_finish+0xb9/0x160 ... entry_SYSCALL_64_after_hwframe+0x77/0x7f read to 0xffff8a7c44c59620 of 8 bytes by task 549 on cpu 14: rt6_nh_dump_exceptions+0xb3/0x260 rt6_dump_route+0x53e/0x5f0 fib6_dump_node+0x6d/0xf0 fib6_walk_continue+0x290/0x2d0 fib6_dump_table+0x28d/0x360 inet6_dump_fib+0x37d/0x620 rtnl_dumpit+0x7b/0xd0 netlink_dump+0x3ae/0x7e0 ... entry_SYSCALL_64_after_hwframe+0x77/0x7f 4 locks held by dumper/549: ... #1: (rcu_read_lock){....}-{1:3}, at: inet6_dump_fib+0x88/0x620 #2: (&tb->tb6_lock){+.-.}-{3:3}, at: fib6_dump_table+0x1e9/0x360 #3: (rcu_read_lock){....}-{1:3}, at: rt6_dump_route+0x483/0x5f0 value changed: 0xffff8a7c44e05700 -> 0xffff8a7c45d60100 Reported by Kernel Concurrency Sanitizer on: CPU: 14 UID: 0 PID: 549 Comm: dumper Not tainted 7.2.0-rc7-virtme #38 PREEMPT(lazy) ... Use hlist_for_each_entry_rcu() to safely iterate over the exception list. Fixes: 1e47b4837f3b ("ipv6: Dump route exceptions if requested") Cc: stable@vger.kernel.org Signed-off-by: Yuyang Huang Reviewed-by: Stefano Brivio Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/20260815084651.69477-1-sigefriedhyy@gmail.com Signed-off-by: David S. Miller Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman

ipv6: rpl: fix NULL dereference of idev in ipv6_rpl_srh_rcv()

2026-09-07T15:22:39+00:00

commit f826df95332c07380206dbd54178b6eefb311aba upstream. ipv6_rpl_srh_rcv() dereferences idev from __in6_dev_get() without a NULL check when reading idev->cnf.rpl_seg_enabled. When the device's MTU drops below IPV6_MIN_MTU, addrconf_ifdown() clears dev->ip6_ptr through RCU_INIT_POINTER(). A packet that passed the idev check in ip6_rcv_core() can then reach ipv6_rpl_srh_rcv() with dev->ip6_ptr already NULL. Reproduced by flooding the receiving interface with ping6 traffic while flapping its MTU between 1500 and 1200: BUG: KASAN: null-ptr-deref in ipv6_rpl_srh_rcv+0xb3/0x1070 Read of size 4 at addr 00000000000006b4 by task ping6/394 CPU: 2 UID: 0 PID: 394 Comm: ping6 Not tainted 7.2.0-rc7-micro-vm-dev-00095-g24ef02f934ee #240 PREEMPT(full) Call Trace: kasan_report+0xc6/0x100 ipv6_rpl_srh_rcv+0xb3/0x1070 ip6_protocol_deliver_rcu+0x759/0x9a0 ip6_input_finish+0xa8/0x1b0 ip6_input+0xe1/0x490 ipv6_rcv+0x33d/0x460 __netif_receive_skb_one_core+0xd6/0x130 process_backlog+0x2cc/0xa00 __napi_poll.constprop.0+0x56/0x270 net_rx_action+0x327/0x730 handle_softirqs+0x11e/0x630 do_softirq+0xb3/0xf0 Both ipv6_rpl_srh_rcv() and ipv6_srh_rcv() are called only from ipv6_rthdr_rcv(), which already has an idev lookup. Fix the NULL dereference on the RPL path by checking idev in ipv6_rthdr_rcv(), before it calls either function. The callees take idev as an argument and no longer call __in6_dev_get(), so the packet is now dropped in one place, with SKB_DROP_REASON_IPV6DISABLED on both paths. Fixes: 8610c7c6e3bd ("net: ipv6: add support for rpl sr exthdr") Cc: stable@vger.kernel.org Signed-off-by: Andrea Mayer Tested-by: Xiang Mei Reviewed-by: Simon Horman Link: https://patch.msgid.link/20260817132644.2223-1-andrea.mayer@uniroma2.it Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman