[PATCH net-next 04/11] gre: make gre_parse_header() report a drop reason
From: Anton Danilov <hidden>
Date: 2026-08-31 21:52:01
Also in:
linux-kselftest, lkml
Subsystem:
networking [general], networking [ipv4/ipv6], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, David Ahern, Ido Schimmel, Linus Torvalds
gre_parse_header() returns -EINVAL for six different reasons and its callers turn that into a plain kfree_skb(). The only detail they could get so far was the csum_err flag, which none of them actually reads: both ip_gre and ip6_gre declare it, pass it in and then ignore it. Replace that dead output parameter with an enum skb_drop_reason one and let the two receive paths report what happened. Two reasons are added: - SKB_DROP_REASON_GRE_INVALID_HDR, for a header carrying an unsupported version or the routing bit, - SKB_DROP_REASON_GRE_CSUM, for a checksum error, next to the existing TCP_CSUM, UDP_CSUM, ICMP_CSUM and IP_CSUM. The header pull failures reuse SKB_DROP_REASON_HDR_TRUNC, which documents exactly this case, and gre_rcv() in the demux reuses pskb_may_pull_reason() and SKB_DROP_REASON_UNHANDLED_PROTO. A NULL reason keeps the meaning a NULL csum_err had: the caller is not interested in it and the checksum must not be verified. This matters for the ICMP error handlers, which only get a part of the original packet and must not drop it when the checksum does not validate. Tunnel lookup failures still report SKB_DROP_REASON_NOT_SPECIFIED here; they are addressed in the following patches. Assisted-by: Claude-Code:claude-opus-5 Signed-off-by: Anton Danilov <redacted> --- include/net/dropreason-core.h | 9 +++++++ include/net/gre.h | 2 +- net/ipv4/gre_demux.c | 51 ++++++++++++++++++++++++++--------- net/ipv4/ip_gre.c | 6 ++--- net/ipv6/ip6_gre.c | 6 ++--- 5 files changed, 55 insertions(+), 19 deletions(-)
diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 40f94ecb912a..8efa682601f0 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h@@ -130,6 +130,8 @@ FN(RECURSION_LIMIT) \ FN(IP_TUNNEL_CFG_OPTS_MISMATCH) \ FN(IP_TUNNEL_OLD_SEQ) \ + FN(GRE_INVALID_HDR) \ + FN(GRE_CSUM) \ FNe(MAX) /**
@@ -622,6 +624,13 @@ enum skb_drop_reason { * numbering. */ SKB_DROP_REASON_IP_TUNNEL_OLD_SEQ, + /** + * @SKB_DROP_REASON_GRE_INVALID_HDR: the GRE header is invalid, e.g. + * an unsupported version or the routing bit is set. + */ + SKB_DROP_REASON_GRE_INVALID_HDR, + /** @SKB_DROP_REASON_GRE_CSUM: GRE checksum error */ + SKB_DROP_REASON_GRE_CSUM, /** * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which * shouldn't be used as a real 'reason' - only for tracing code gen
diff --git a/include/net/gre.h b/include/net/gre.h
index b55f67ecd2fc..a63f26c3f78e 100644
--- a/include/net/gre.h
+++ b/include/net/gre.h@@ -33,7 +33,7 @@ int gre_add_protocol(const struct gre_protocol *proto, u8 version); int gre_del_protocol(const struct gre_protocol *proto, u8 version); int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi, - bool *csum_err, __be16 proto, int nhs); + enum skb_drop_reason *reason, __be16 proto, int nhs); static inline bool netif_is_gretap(const struct net_device *dev) {
diff --git a/net/ipv4/gre_demux.c b/net/ipv4/gre_demux.c
index 96fd7dc6d82d..598678f42ef5 100644
--- a/net/ipv4/gre_demux.c
+++ b/net/ipv4/gre_demux.c@@ -58,26 +58,43 @@ EXPORT_SYMBOL_GPL(gre_del_protocol); /* Fills in tpi and returns header length to be pulled. * Note that caller must use pskb_may_pull() before pulling GRE header. + * + * @reason is only written when the header is rejected, so the caller has + * to initialise it before the call. + * + * A NULL @reason means that the caller is not interested in the drop + * reason, and also that the checksum must not be verified. This is what + * the ICMP error handlers need, as they only get a part of the original + * packet. */ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi, - bool *csum_err, __be16 proto, int nhs) + enum skb_drop_reason *reason, __be16 proto, int nhs) { const struct gre_base_hdr *greh; __be32 *options; int hdr_len; - if (unlikely(!pskb_may_pull(skb, nhs + sizeof(struct gre_base_hdr)))) + if (unlikely(!pskb_may_pull(skb, nhs + sizeof(struct gre_base_hdr)))) { + if (reason) + *reason = SKB_DROP_REASON_HDR_TRUNC; return -EINVAL; + } greh = (struct gre_base_hdr *)(skb->data + nhs); - if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING))) + if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING))) { + if (reason) + *reason = SKB_DROP_REASON_GRE_INVALID_HDR; return -EINVAL; + } gre_flags_to_tnl_flags(tpi->flags, greh->flags); hdr_len = gre_calc_hlen(tpi->flags); - if (!pskb_may_pull(skb, nhs + hdr_len)) + if (!pskb_may_pull(skb, nhs + hdr_len)) { + if (reason) + *reason = SKB_DROP_REASON_HDR_TRUNC; return -EINVAL; + } greh = (struct gre_base_hdr *)(skb->data + nhs); tpi->proto = greh->protocol;
@@ -87,8 +104,8 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi, if (!skb_checksum_simple_validate(skb)) { skb_checksum_try_convert(skb, IPPROTO_GRE, null_compute_pseudo); - } else if (csum_err) { - *csum_err = true; + } else if (reason) { + *reason = SKB_DROP_REASON_GRE_CSUM; return -EINVAL; }
@@ -116,8 +133,11 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi, val = skb_header_pointer(skb, nhs + hdr_len, sizeof(_val), &_val); - if (!val) + if (!val) { + if (reason) + *reason = SKB_DROP_REASON_HDR_TRUNC; return -EINVAL; + } tpi->proto = proto; if ((*val & 0xF0) != 0x40) hdr_len += 4;
@@ -132,8 +152,11 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi, greh->protocol == htons(ETH_P_ERSPAN2)) { struct erspan_base_hdr *ershdr; - if (!pskb_may_pull(skb, nhs + hdr_len + sizeof(*ershdr))) + if (!pskb_may_pull(skb, nhs + hdr_len + sizeof(*ershdr))) { + if (reason) + *reason = SKB_DROP_REASON_HDR_TRUNC; return -EINVAL; + } ershdr = (struct erspan_base_hdr *)(skb->data + nhs + hdr_len); tpi->key = cpu_to_be32(get_session_id(ershdr));
@@ -146,15 +169,19 @@ EXPORT_SYMBOL(gre_parse_header); static int gre_rcv(struct sk_buff *skb) { const struct gre_protocol *proto; + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; u8 ver; int ret; - if (!pskb_may_pull(skb, 12)) + reason = pskb_may_pull_reason(skb, 12); + if (reason) goto drop; ver = skb->data[1]&0x7f; - if (ver >= GREPROTO_MAX) + if (ver >= GREPROTO_MAX) { + reason = SKB_DROP_REASON_UNHANDLED_PROTO; goto drop; + } rcu_read_lock(); proto = rcu_dereference(gre_proto[ver]);
@@ -167,11 +194,11 @@ static int gre_rcv(struct sk_buff *skb) drop_nohandler: rcu_read_unlock(); dev_core_stats_rx_nohandler_inc(skb->dev); - kfree_skb(skb); + kfree_skb_reason(skb, SKB_DROP_REASON_UNHANDLED_PROTO); return NET_RX_DROP; drop: dev_core_stats_rx_dropped_inc(skb->dev); - kfree_skb(skb); + kfree_skb_reason(skb, reason); return NET_RX_DROP; }
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 82309efd417e..1894c5746a73 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c@@ -439,8 +439,8 @@ static int ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi, static int gre_rcv(struct sk_buff *skb) { + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; struct tnl_ptk_info tpi; - bool csum_err = false; int hdr_len; #ifdef CONFIG_NET_IPGRE_BROADCAST
@@ -451,7 +451,7 @@ static int gre_rcv(struct sk_buff *skb) } #endif - hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IP), 0); + hdr_len = gre_parse_header(skb, &tpi, &reason, htons(ETH_P_IP), 0); if (hdr_len < 0) goto drop;
@@ -469,7 +469,7 @@ static int gre_rcv(struct sk_buff *skb) icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0); drop: dev_core_stats_rx_dropped_inc(skb->dev); - kfree_skb(skb); + kfree_skb_reason(skb, reason); return 0; }
diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 69c51f1a5bf0..736eefdb528f 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c@@ -569,11 +569,11 @@ static int ip6erspan_rcv(struct sk_buff *skb, static int gre_rcv(struct sk_buff *skb) { + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; struct tnl_ptk_info tpi; - bool csum_err = false; int hdr_len; - hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IPV6), 0); + hdr_len = gre_parse_header(skb, &tpi, &reason, htons(ETH_P_IPV6), 0); if (hdr_len < 0) goto drop;
@@ -594,7 +594,7 @@ static int gre_rcv(struct sk_buff *skb) icmpv6_send(skb, ICMPV6_DEST_UNREACH, ICMPV6_PORT_UNREACH, 0); drop: dev_core_stats_rx_dropped_inc(skb->dev); - kfree_skb(skb); + kfree_skb_reason(skb, reason); return 0; }
--
2.47.3