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Wed, 28 Oct 2026 16:17:17 -0700 (PDT) Received: from macbook-pro-de-joas.local ([2804:7f0:b101:c5e8:7d75:cb37:7054:6d4b]) by smtp.gmail.com with ESMTPSA id ada2fe7eead31-7cea3d7dd7esm7234241137.10.2026.10.28.16.17.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Oct 2026 16:17:16 -0700 (PDT) From: Joas Antonio dos Santos To: Jon Maloy , Tung Quang Nguyen Cc: David S. Miller , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org, tipc-discussion@lists.sourceforge.net, linux-kernel@vger.kernel.org Subject: [PATCH net v4] tipc: raise the minimum bearer MTU to fit tunnel and crypto overhead Date: Wed, 28 Oct 2026 20:17:06 -0300 Message-ID: <179322942644.17111.4355153262478871713@gmail.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 A peer can lower the link MTU to TIPC_MIN_BEARER_MTU (100 bytes, plus the UDP encapsulation header) with the max_pkt field of a RESET or ACTIVATE message; tipc_link_proto_rcv() only checks the lower bound returned by tipc_bearer_min_mtu(). The same bound is applied to the device MTU when a bearer is enabled. That bound predates two later reductions of the link MSS: tipc_link_mss() = mtu - INT_H_SIZE (tunnel header) - EMSG_OVERHEAD (CONFIG_TIPC_CRYPTO) With CONFIG_TIPC_CRYPTO=y, which is the default, an MTU of 100 gives an MSS of 28. Once the link comes up, tipc_named_node_up() calls named_distribute(), which computes msg_dsz = ((tipc_node_get_mtu() - INT_H_SIZE) / ITEM_SIZE) * ITEM_SIZE; For an MSS of 28 the subtraction is negative and the result truncates to 0xfffffff0; for an MSS between 40 and 59 it is 0. named_prepare_buf() then allocates a buffer with no room for any item (the u32 size wraps), and msg_rem never reaches zero, so every cluster-scope publication on the node is written with publ_to_item() past the end of that one skb, over skb_shared_info and into adjacent memory. Without crypto the MSS is at least 60, which still does not hold the first fragment built by tipc_msg_build(): INT_H_SIZE fragment header plus a message header of up to MAX_H_SIZE. With pktmax below that, the header copies run past the pktmax bytes allocated for the fragment. Raise TIPC_MIN_BEARER_MTU to MAX_H_SIZE + 2 * INT_H_SIZE so the MSS can always hold INT_H_SIZE + MAX_H_SIZE, which also leaves room for name table items, and add tipc_bearer_mtu_floor(), which adds EMSG_OVERHEAD on top for CONFIG_TIPC_CRYPTO=y. Use it for the device MTU check (tipc_mtu_bad()), the UDP MTU check, the netlink MTU setting and tipc_bearer_min_mtu(), which also bounds the max_pkt negotiation. Found by manual code review, assisted by an LLM, and reproduced with KASAN using two network namespaces joined by a veth pair (see below). Fixes: fc1b6d6de220 ("tipc: introduce TIPC encryption & authentication") Fixes: 2320bcdae628 ("tipc: fix changeover issues due to large packet") Suggested-by: Tung Nguyen Signed-off-by: Joas Antonio dos Santos Assisted-by: Claude:claude-opus-5-5 --- Changes in v4: - Rebased on net/main, on top of 592d0fdb02df ("tipc: bound the device MTU accepted for L2 bearers"); tipc_mtu_bad() keeps the new TIPC_MAX_BEARER_MTU upper bound and only the lower bound changes. No functional change otherwise. Changes in v3: - Keep the CONFIG_TIPC_CRYPTO dependency in one helper, tipc_bearer_mtu_floor(), instead of #ifdefs at each call site, as suggested by Paolo. tipc_mtu_bad() and tipc_udp_mtu_bad() use it, so tipc_mtu_bad_crypto() and the crypto.h include in udp_media.h are gone. The bounds are unchanged: 172 with crypto, 140 without. Changes in v2: - Derive the crypto bound from EMSG_OVERHEAD instead of a hard-coded 32: TIPC_MIN_BEARER_MTU_CRYPTO in crypto.h and tipc_mtu_bad_crypto(), as suggested by Tung. Drop the BUILD_BUG_ON(). - Apply the crypto bound to the UDP MTU check and the netlink MTU setting too. v4 builds cleanly on net/main with W=1, with and without CONFIG_TIPC_CRYPTO. The runtime results below are from v3, retested on 602042bf29f6 (arm64, QEMU, KASAN); the rebase does not change the bounds. With the reproducer below (node A with 40 cluster-scope publications and MTU 1500, node B enabling its bearer on a veth with the given MTU): config peer MTU bearer on B link KASAN unpatched 100 enabled up slab-out-of-bounds in publ_to_item() TIPC_CRYPTO=y 100 refused (too low) - none TIPC_CRYPTO=y 171 refused (too low) - none TIPC_CRYPTO=y 172 enabled up none TIPC_CRYPTO=y 1500 enabled up none TIPC_CRYPTO=n 100 refused (too low) - none TIPC_CRYPTO=n 139 refused (too low) - none TIPC_CRYPTO=n 140 enabled up none TIPC_CRYPTO=n 1500 enabled up none The reproducer exercises the bearer-enable check; the max_pkt negotiation in tipc_link_proto_rcv() uses the same bound through tipc_bearer_min_mtu() but was not hit separately with forged messages. Reproducer (as root; build with "gcc -Wall -O2 -o pub pub.c", run "sh tipc_repro.sh [peer_mtu]"): pub.c: /* Bind N TIPC services with cluster scope, then sleep. */ #include #include #include #include #include int main(int argc, char **argv) { int n = argc > 1 ? atoi(argv[1]) : 40; for (int i = 0; i < n; i++) { struct sockaddr_tipc a = { .family = AF_TIPC, .addrtype = TIPC_SERVICE_RANGE, .scope = TIPC_CLUSTER_SCOPE, .addr.nameseq = { .type = 1000 + i, .lower = 0, .upper = 0 }, }; int s = socket(AF_TIPC, SOCK_RDM, 0); if (s < 0 || bind(s, (struct sockaddr *)&a, sizeof(a))) { perror("tipc bind"); return 1; } } printf("pub: bound %d cluster-scope services\n", n); fflush(stdout); pause(); return 0; } tipc_repro.sh: #!/bin/sh # TIPC named_distribute() overflow reproducer. # Needs root, CONFIG_TIPC=y, CONFIG_TIPC_CRYPTO=y (default), KASAN recommended. # Usage: ./tipc_repro.sh [peer_mtu] (default 100; 100..131 trigger it) PEER_MTU=${1:-100} ip netns add A ip netns add B ip link add va type veth peer name vb ip link set va netns A ip link set vb netns B ip -n A link set lo up ip -n B link set lo up ip -n A link set va mtu 1500 up ip -n B link set vb mtu "$PEER_MTU" up # node A: 40 cluster-scope publications (any number >= 1 overflows; # more publications write further past the buffer) ip netns exec A ./pub 40 & sleep 2 # node A uses MTU 1500, node B uses PEER_MTU; the link MTU is # negotiated down to PEER_MTU, the link MSS becomes PEER_MTU - 72 ip netns exec A tipc bearer enable media eth dev va ip netns exec B tipc bearer enable media eth dev vb # the overflow happens in tipc_named_node_up() as soon as the link # comes up (a few seconds) sleep 20 dmesg | grep -A30 -E "KASAN|BUG:|Oops" net/tipc/bearer.c | 17 +++++++++++++++-- net/tipc/bearer.h | 9 ++++++--- net/tipc/udp_media.h | 2 +- 3 files changed, 22 insertions(+), 6 deletions(-) diff --git a/net/tipc/bearer.c b/net/tipc/bearer.c index 774f68d..8b82101 100644 --- a/net/tipc/bearer.c +++ b/net/tipc/bearer.c @@ -546,9 +546,22 @@ int tipc_bearer_mtu(struct net *net, u32 bearer_id) return mtu; } +/* tipc_bearer_mtu_floor - smallest bearer MTU, excluding the media + * encapsulation header, that leaves room for a fragment header plus a + * full message header after the link MSS reductions (see tipc_link_mss()) + */ +u32 tipc_bearer_mtu_floor(void) +{ +#ifdef CONFIG_TIPC_CRYPTO + return TIPC_MIN_BEARER_MTU + EMSG_OVERHEAD; +#else + return TIPC_MIN_BEARER_MTU; +#endif +} + int tipc_bearer_min_mtu(struct net *net, u32 bearer_id) { - int mtu = TIPC_MIN_BEARER_MTU; + int mtu = tipc_bearer_mtu_floor(); struct tipc_bearer *b; rcu_read_lock(); @@ -1164,7 +1177,7 @@ int __tipc_nl_bearer_set(struct sk_buff *skb, struct genl_info *info) } #ifdef CONFIG_TIPC_MEDIA_UDP if (nla_get_u32(props[TIPC_NLA_PROP_MTU]) < - b->encap_hlen + TIPC_MIN_BEARER_MTU) { + b->encap_hlen + tipc_bearer_mtu_floor()) { NL_SET_ERR_MSG(info->extack, "MTU value is out-of-range"); return -EINVAL; diff --git a/net/tipc/bearer.h b/net/tipc/bearer.h index f9bfcc9..72abfca 100644 --- a/net/tipc/bearer.h +++ b/net/tipc/bearer.h @@ -60,8 +60,10 @@ #define TIPC_MEDIA_TYPE_IB 2 #define TIPC_MEDIA_TYPE_UDP 3 -/* Minimum bearer MTU */ -#define TIPC_MIN_BEARER_MTU (MAX_H_SIZE + INT_H_SIZE) +/* Minimum bearer MTU, not counting the crypto overhead, see + * tipc_bearer_mtu_floor() + */ +#define TIPC_MIN_BEARER_MTU (MAX_H_SIZE + 2 * INT_H_SIZE) /* Maximum bearer MTU * @@ -241,6 +243,7 @@ void tipc_bearer_cleanup(void); void tipc_bearer_stop(struct net *net); int tipc_bearer_mtu(struct net *net, u32 bearer_id); int tipc_bearer_min_mtu(struct net *net, u32 bearer_id); +u32 tipc_bearer_mtu_floor(void); bool tipc_bearer_bcast_support(struct net *net, u32 bearer_id); void tipc_bearer_xmit_skb(struct net *net, u32 bearer_id, struct sk_buff *skb, @@ -265,7 +268,7 @@ static inline void tipc_loopback_trace(struct net *net, /* check if device MTU is usable for tipc bearers */ static inline bool tipc_mtu_bad(struct net_device *dev) { - if (dev->mtu >= TIPC_MIN_BEARER_MTU && + if (dev->mtu >= tipc_bearer_mtu_floor() && dev->mtu <= TIPC_MAX_BEARER_MTU) return false; netdev_warn(dev, "MTU not usable for tipc bearer\n"); diff --git a/net/tipc/udp_media.h b/net/tipc/udp_media.h index e7455cc..030e00a 100644 --- a/net/tipc/udp_media.h +++ b/net/tipc/udp_media.h @@ -48,7 +48,7 @@ int tipc_udp_nl_dump_remoteip(struct sk_buff *skb, struct netlink_callback *cb); /* check if configured MTU is too low for tipc headers */ static inline bool tipc_udp_mtu_bad(u32 mtu) { - if (mtu >= (TIPC_MIN_BEARER_MTU + sizeof(struct iphdr) + + if (mtu >= (tipc_bearer_mtu_floor() + sizeof(struct iphdr) + sizeof(struct udphdr))) return false; -- 2.39.5