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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , David Ahern , Simon Horman Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, David Lebrun , Hangbin Liu , Stefano Salsano , Andrea Mayer Subject: [PATCH net] seg6: ensure packet data is writable before modifying SRH and IPv6 DA Date: Fri, 25 Sep 2026 15:38:07 +0200 Message-Id: <20260925133807.32-1-andrea.mayer@uniroma2.it> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Virus-Scanned: clamav-milter 0.100.0 at smtp-2015 X-Virus-Status: Clean advance_nextseg() modifies the SRH Segments Left field and the IPv6 destination address without ensuring the packet data is writable. seg6_next_csid_advance_arg() has the same problem when it advances the NEXT-C-SID argument in the destination address. The skb may be cloned, for example by an AF_PACKET socket receiving on the ingress device. advance_nextseg() and seg6_next_csid_advance_arg() then write into the packet data shared with the clone. A read from that socket can return the modified packet instead of the received one. The simplified path below shows this for advance_nextseg(): __netif_receive_skb_one_core __netif_receive_skb_core deliver_skb [orig: users=2, cloned=0] packet_rcv skb_clone clone queued to the AF_PACKET socket consume_skb(orig) [orig: users=1, cloned=1] ipv6_rcv ip6_rcv_core skb_share_check: no-op [orig: users=1] [...] input_action_end_core advance_nextseg writes into the data shared with the clone Call skb_ensure_writable() in advance_nextseg() and in seg6_next_csid_advance_arg() before they modify the packet data. skb_ensure_writable() may reallocate skb->head, which invalidates the pointers into the packet data taken before the call. advance_nextseg() now returns a valid SRH pointer, or NULL if skb_ensure_writable() fails. seg6_next_csid_advance_arg() takes the pointer to the destination address from the skb after skb_ensure_writable(). On failure, the callers drop the packet with SKB_DROP_REASON_NOMEM. Fixes: 140f04c33bbc ("ipv6: sr: implement several seg6local actions") Fixes: 848f3c0d4769 ("seg6: add NEXT-C-SID support for SRv6 End behavior") Signed-off-by: Andrea Mayer --- net/ipv6/seg6_local.c | 140 +++++++++++++++++++++++++++++++++++------- 1 file changed, 117 insertions(+), 23 deletions(-) diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c index d1070aec7b72..53f7b2452575 100644 --- a/net/ipv6/seg6_local.c +++ b/net/ipv6/seg6_local.c @@ -278,13 +278,43 @@ static bool decap_and_validate(struct sk_buff *skb, int proto) return true; } -static void advance_nextseg(struct ipv6_sr_hdr *srh, struct in6_addr *daddr) +/* advance the SRH to the next segment and set the IPv6 DA accordingly. + * skb pointers may change: after this call, the caller must evaluate again + * any pointer into the packet data. + * + * This function returns: + * - the SRH on success; + * - NULL when the skb cannot be made writable. In this case, the function + * sets the drop reason to SKB_DROP_REASON_NOMEM. + */ +static struct ipv6_sr_hdr *advance_nextseg(struct sk_buff *skb, + struct ipv6_sr_hdr *srh, + enum skb_drop_reason *reason) { struct in6_addr *addr; + int srhoff; + int wlen; + + srhoff = (unsigned char *)srh - skb->data; + /* we write only the Segment Left field and the IPv6 DA. The segment + * list is only read, and seg6_get_srh() already pulled it into the + * linear area. + */ + wlen = srhoff + sizeof(*srh); + + if (unlikely(skb_ensure_writable(skb, wlen))) { + *reason = SKB_DROP_REASON_NOMEM; + return NULL; + } + + /* skb_ensure_writable() may change skb pointers; evaluate srh again */ + srh = (struct ipv6_sr_hdr *)(skb->data + srhoff); srh->segments_left--; addr = srh->segments + srh->segments_left; - *daddr = *addr; + ipv6_hdr(skb)->daddr = *addr; + + return srh; } static int @@ -382,12 +412,26 @@ static bool seg6_next_csid_is_arg_zero(const struct in6_addr *addr, return true; } -/* assume that DA.Argument length > 0 */ -static void seg6_next_csid_advance_arg(struct in6_addr *addr, - const struct seg6_flavors_info *finfo) +/* assume that DA.Argument length > 0. + * skb pointers may change: after this call, the caller must evaluate again + * any pointer into the packet data. + * + * This function returns: + * - SKB_NOT_DROPPED_YET on success; + * - SKB_DROP_REASON_NOMEM when the skb cannot be made writable. + */ +static enum skb_drop_reason +seg6_next_csid_advance_arg(struct sk_buff *skb, + const struct seg6_flavors_info *finfo) { __u8 fnc_octects = seg6_flv_lcnode_func_octects(finfo); __u8 blk_octects = seg6_flv_lcblock_octects(finfo); + struct in6_addr *addr; + + if (unlikely(skb_ensure_writable(skb, sizeof(struct ipv6hdr)))) + return SKB_DROP_REASON_NOMEM; + + addr = &ipv6_hdr(skb)->daddr; /* advance DA.Argument */ memmove(&addr->s6_addr[blk_octects], @@ -395,6 +439,8 @@ static void seg6_next_csid_advance_arg(struct in6_addr *addr, 16 - blk_octects - fnc_octects); memset(&addr->s6_addr[16 - fnc_octects], 0x00, fnc_octects); + + return SKB_NOT_DROPPED_YET; } static int input_action_end_finish(struct sk_buff *skb, @@ -408,31 +454,42 @@ static int input_action_end_finish(struct sk_buff *skb, static int input_action_end_core(struct sk_buff *skb, struct seg6_local_lwt *slwt) { + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; struct ipv6_sr_hdr *srh; + int err = -EINVAL; srh = get_and_validate_srh(skb); if (!srh) goto drop; - advance_nextseg(srh, &ipv6_hdr(skb)->daddr); + srh = advance_nextseg(skb, srh, &reason); + if (!srh) { + err = -ENOMEM; + goto drop; + } return input_action_end_finish(skb, slwt); drop: - kfree_skb(skb); - return -EINVAL; + kfree_skb_reason(skb, reason); + return err; } static int end_next_csid_core(struct sk_buff *skb, struct seg6_local_lwt *slwt) { const struct seg6_flavors_info *finfo = &slwt->flv_info; struct in6_addr *daddr = &ipv6_hdr(skb)->daddr; + enum skb_drop_reason reason; if (seg6_next_csid_is_arg_zero(daddr, finfo)) return input_action_end_core(skb, slwt); /* update DA */ - seg6_next_csid_advance_arg(daddr, finfo); + reason = seg6_next_csid_advance_arg(skb, finfo); + if (reason) { + kfree_skb_reason(skb, reason); + return -ENOMEM; + } return input_action_end_finish(skb, slwt); } @@ -448,19 +505,25 @@ static int input_action_end_x_finish(struct sk_buff *skb, static int input_action_end_x_core(struct sk_buff *skb, struct seg6_local_lwt *slwt) { + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; struct ipv6_sr_hdr *srh; + int err = -EINVAL; srh = get_and_validate_srh(skb); if (!srh) goto drop; - advance_nextseg(srh, &ipv6_hdr(skb)->daddr); + srh = advance_nextseg(skb, srh, &reason); + if (!srh) { + err = -ENOMEM; + goto drop; + } return input_action_end_x_finish(skb, slwt); drop: - kfree_skb(skb); - return -EINVAL; + kfree_skb_reason(skb, reason); + return err; } static int end_x_next_csid_core(struct sk_buff *skb, @@ -468,12 +531,17 @@ static int end_x_next_csid_core(struct sk_buff *skb, { const struct seg6_flavors_info *finfo = &slwt->flv_info; struct in6_addr *daddr = &ipv6_hdr(skb)->daddr; + enum skb_drop_reason reason; if (seg6_next_csid_is_arg_zero(daddr, finfo)) return input_action_end_x_core(skb, slwt); /* update DA */ - seg6_next_csid_advance_arg(daddr, finfo); + reason = seg6_next_csid_advance_arg(skb, finfo); + if (reason) { + kfree_skb_reason(skb, reason); + return -ENOMEM; + } return input_action_end_x_finish(skb, slwt); } @@ -760,10 +828,12 @@ static bool seg6_pop_srh(struct sk_buff *skb, int srhoff) */ static int end_flv8986_core(struct sk_buff *skb, struct seg6_local_lwt *slwt) { + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; const struct seg6_flavors_info *finfo = &slwt->flv_info; enum seg6_local_flv_action action; enum seg6_local_pktinfo pinfo; struct ipv6_sr_hdr *srh; + int err = -EINVAL; __u32 flvmask; int srhoff; @@ -787,10 +857,18 @@ static int end_flv8986_core(struct sk_buff *skb, struct seg6_local_lwt *slwt) switch (action) { case SEG6_LOCAL_FLV_ACT_END: /* process the packet as the "standard" End behavior */ - advance_nextseg(srh, &ipv6_hdr(skb)->daddr); + srh = advance_nextseg(skb, srh, &reason); + if (!srh) { + err = -ENOMEM; + goto drop; + } break; case SEG6_LOCAL_FLV_ACT_PSP: - advance_nextseg(srh, &ipv6_hdr(skb)->daddr); + srh = advance_nextseg(skb, srh, &reason); + if (!srh) { + err = -ENOMEM; + goto drop; + } if (unlikely(!seg6_pop_srh(skb, srhoff))) goto drop; @@ -807,8 +885,8 @@ static int end_flv8986_core(struct sk_buff *skb, struct seg6_local_lwt *slwt) return input_action_end_finish(skb, slwt); drop: - kfree_skb(skb); - return -EINVAL; + kfree_skb_reason(skb, reason); + return err; } /* regular endpoint function */ @@ -847,21 +925,27 @@ static int input_action_end_x(struct sk_buff *skb, struct seg6_local_lwt *slwt) static int input_action_end_t(struct sk_buff *skb, struct seg6_local_lwt *slwt) { + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; struct ipv6_sr_hdr *srh; + int err = -EINVAL; srh = get_and_validate_srh(skb); if (!srh) goto drop; - advance_nextseg(srh, &ipv6_hdr(skb)->daddr); + srh = advance_nextseg(skb, srh, &reason); + if (!srh) { + err = -ENOMEM; + goto drop; + } seg6_lookup_nexthop(skb, NULL, slwt->table); return dst_input(skb); drop: - kfree_skb(skb); - return -EINVAL; + kfree_skb_reason(skb, reason); + return err; } /* decapsulate and forward inner L2 frame on specified interface */ @@ -1375,6 +1459,7 @@ static int input_action_end_b6(struct sk_buff *skb, struct seg6_local_lwt *slwt) static int input_action_end_b6_encap(struct sk_buff *skb, struct seg6_local_lwt *slwt) { + enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED; struct ipv6_sr_hdr *srh; int err = -EINVAL; @@ -1382,7 +1467,11 @@ static int input_action_end_b6_encap(struct sk_buff *skb, if (!srh) goto drop; - advance_nextseg(srh, &ipv6_hdr(skb)->daddr); + srh = advance_nextseg(skb, srh, &reason); + if (!srh) { + err = -ENOMEM; + goto drop; + } skb_reset_inner_headers(skb); skb->encapsulation = 1; @@ -1398,7 +1487,7 @@ static int input_action_end_b6_encap(struct sk_buff *skb, return dst_input(skb); drop: - kfree_skb(skb); + kfree_skb_reason(skb, reason); return err; } @@ -1434,6 +1523,7 @@ static int input_action_end_bpf(struct sk_buff *skb, struct seg6_local_lwt *slwt) { struct seg6_bpf_srh_state *srh_state; + enum skb_drop_reason reason; struct ipv6_sr_hdr *srh; int ret; @@ -1442,7 +1532,11 @@ static int input_action_end_bpf(struct sk_buff *skb, kfree_skb(skb); return -EINVAL; } - advance_nextseg(srh, &ipv6_hdr(skb)->daddr); + srh = advance_nextseg(skb, srh, &reason); + if (!srh) { + kfree_skb_reason(skb, reason); + return -ENOMEM; + } /* The access to the per-CPU buffer srh_state is protected by running * always in softirq context (with disabled BH). On PREEMPT_RT the base-commit: 11536ee3d3e0b1bd35b6f3f8df55a6053eb0c71d -- 2.43.0