From 3744b0964d5267c0b651bcd8f8c25db6bf4ccbac Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Wed, 29 Apr 2026 17:46:48 +0200 Subject: ipv6: Implement limits on extension header parsing ipv6_{skip_exthdr,find_hdr}() and ip6_{tnl_parse_tlv_enc_lim, protocol_deliver_rcu}() iterate over IPv6 extension headers until they find a non-extension-header protocol or run out of packet data. The loops have no iteration counter, relying solely on the packet length to bound them. For a crafted packet with 8-byte extension headers filling a 64KB jumbogram, this means a worst case of up to ~8k iterations with a skb_header_pointer call each. ipv6_skip_exthdr(), for example, is used where it parses the inner quoted packet inside an incoming ICMPv6 error: - icmpv6_rcv - checksum validation - case ICMPV6_DEST_UNREACH - icmpv6_notify - pskb_may_pull() <- pull inner IPv6 header - ipv6_skip_exthdr() <- iterates here - pskb_may_pull() - ipprot->err_handler() <- sk lookup The per-iteration cost of ipv6_skip_exthdr itself is generally light, but skb_header_pointer becomes more costly on reassembled packets: the first ~1232 bytes of the inner packet are in the skb's linear area, but the remaining ~63KB are in the frag_list where skb_copy_bits is needed to read data. Initially, the idea was to add a configurable limit via a new sysctl knob with default 8, in line with knobs from commit 47d3d7ac656a ("ipv6: Implement limits on Hop-by-Hop and Destination options"), but two reasons eventually argued against it: - It adds to UAPI that needs to be maintained forever, and upcoming work is restricting extension header ordering anyway, leaving little reason for another sysctl knob - exthdrs_core.c is always built-in even when CONFIG_IPV6=n, where struct net has no .ipv6 member, so the read site would need an ifdef'd fallback to a constant anyway Therefore, just use a constant (IP6_MAX_EXT_HDRS_CNT). All four extension header walking functions are now bound by this limit. Note that the check in ip6_protocol_deliver_rcu() happens right before the goto resubmit, such that we don't have to have a test for ipv6_ext_hdr() in the fast-path. There's an ongoing IETF draft-iurman-6man-eh-occurrences to enforce IPv6 extension headers ordering and occurrence. The latter also discusses security implications. As per RFC8200 section 4.1, the occurrence rules for extension headers provide a practical upper bound which is 8. In order to be conservative, let's define IP6_MAX_EXT_HDRS_CNT as 12 to leave enough room for quirky setups. In the unlikely event that this is still not enough, then we might need to reconsider a sysctl. Signed-off-by: Daniel Borkmann Reviewed-by: Ido Schimmel Reviewed-by: Eric Dumazet Reviewed-by: Justin Iurman Link: https://patch.msgid.link/20260429154648.809751-1-daniel@iogearbox.net Signed-off-by: Jakub Kicinski --- include/net/dropreason-core.h | 6 ++++++ include/net/ipv6.h | 3 +++ 2 files changed, 9 insertions(+) (limited to 'include') diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h index e0ca3904ff8e..2f312d1f67d6 100644 --- a/include/net/dropreason-core.h +++ b/include/net/dropreason-core.h @@ -99,6 +99,7 @@ FN(FRAG_TOO_FAR) \ FN(TCP_MINTTL) \ FN(IPV6_BAD_EXTHDR) \ + FN(IPV6_TOO_MANY_EXTHDRS) \ FN(IPV6_NDISC_FRAG) \ FN(IPV6_NDISC_HOP_LIMIT) \ FN(IPV6_NDISC_BAD_CODE) \ @@ -494,6 +495,11 @@ enum skb_drop_reason { SKB_DROP_REASON_TCP_MINTTL, /** @SKB_DROP_REASON_IPV6_BAD_EXTHDR: Bad IPv6 extension header. */ SKB_DROP_REASON_IPV6_BAD_EXTHDR, + /** + * @SKB_DROP_REASON_IPV6_TOO_MANY_EXTHDRS: Number of IPv6 extension + * headers in the packet exceeds IP6_MAX_EXT_HDRS_CNT. + */ + SKB_DROP_REASON_IPV6_TOO_MANY_EXTHDRS, /** @SKB_DROP_REASON_IPV6_NDISC_FRAG: invalid frag (suppress_frag_ndisc). */ SKB_DROP_REASON_IPV6_NDISC_FRAG, /** @SKB_DROP_REASON_IPV6_NDISC_HOP_LIMIT: invalid hop limit. */ diff --git a/include/net/ipv6.h b/include/net/ipv6.h index d042afe7a245..1dec81faff28 100644 --- a/include/net/ipv6.h +++ b/include/net/ipv6.h @@ -90,6 +90,9 @@ struct ip_tunnel_info; #define IP6_DEFAULT_MAX_DST_OPTS_LEN INT_MAX /* No limit */ #define IP6_DEFAULT_MAX_HBH_OPTS_LEN INT_MAX /* No limit */ +/* Hard limit on traversed IPv6 extension headers */ +#define IP6_MAX_EXT_HDRS_CNT 12 + /* * Addr type * -- cgit v1.2.3