[PATCH net v3] ipv6: rpl: reserve mac_len headroom when recompressed SRH grows

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

ipv6_rpl_srh_rcv() decompresses an RFC 6554 Source Routing Header, swaps
the next segment into ipv6_hdr->daddr, recompresses, then pulls the old
header and pushes the new one plus the IPv6 header back.  The
recompressed header can be larger than the received one when the swap
reduces the common-prefix length the segments share with daddr (CmprI=0,
CmprE>0, seg[0][0] != daddr[0] gives the maximum +8 bytes).

pskb_expand_head() was gated on segments_left == 0, so on earlier
segments the push consumed unchecked headroom.  Once skb_push() leaves
fewer than skb->mac_len bytes in front of data,
skb_mac_header_rebuild()'s call to:

	skb_set_mac_header(skb, -skb->mac_len);

will store (data - head) - mac_len into the u16 mac_header field, which
wraps to ~65530, and the following memmove() writes mac_len bytes ~64KiB
past skb->head.

A single AF_INET6/SOCK_RAW/IPV6_HDRINCL packet over lo with a two
segment type-3 SRH (CmprI=0, CmprE=15) reaches headroom 8 after one
pass; KASAN reports a 14-byte OOB write in ipv6_rthdr_rcv.

Fix this by expanding the head whenever the remaining room is less than
the push size plus mac_len, and request that much extra so the rebuilt
MAC header fits afterwards.

Fixes: 8610c7c6e3bd ("net: ipv6: add support for rpl sr exthdr")
Cc: "David S. Miller" <davem@davemloft.net>
Cc: David Ahern <dsahern@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: stable <stable@kernel.org>
Reported-by: Anthropic
Assisted-by: gkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
v3: - skb_postpull_rcsum() should not be changed, it's chdr NOT hdr, ugh
      Link to v2: https://lore.kernel.org/r/2026042158-sediment-elliptic-a954@gregkh

v2: - fixed up if statement to actually work properly, and test it
      against a working poc (poc will be sent separately)
      Reworded the changelog and the subject to make more sense
      Link to v1: https://lore.kernel.org/r/2026042024-cabbie-gills-9371@gregkh

 net/ipv6/exthdrs.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index 95558fd6f447..03cbce842c1a 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -491,6 +491,7 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
 	struct net *net = dev_net(skb->dev);
 	struct inet6_dev *idev;
 	struct ipv6hdr *oldhdr;
+	unsigned int chdr_len;
 	unsigned char *buf;
 	int accept_rpl_seg;
 	int i, err;
@@ -592,8 +593,10 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
 	skb_pull(skb, ((hdr->hdrlen + 1) << 3));
 	skb_postpull_rcsum(skb, oldhdr,
 			   sizeof(struct ipv6hdr) + ((hdr->hdrlen + 1) << 3));
-	if (unlikely(!hdr->segments_left)) {
-		if (pskb_expand_head(skb, sizeof(struct ipv6hdr) + ((chdr->hdrlen + 1) << 3), 0,
+	chdr_len = sizeof(struct ipv6hdr) + ((chdr->hdrlen + 1) << 3);
+	if (unlikely(!hdr->segments_left ||
+		     skb_headroom(skb) < chdr_len + skb->mac_len)) {
+		if (pskb_expand_head(skb, chdr_len + skb->mac_len, 0,
 				     GFP_ATOMIC)) {
 			__IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)), IPSTATS_MIB_OUTDISCARDS);
 			kfree_skb(skb);
@@ -603,7 +606,7 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
 
 		oldhdr = ipv6_hdr(skb);
 	}
-	skb_push(skb, ((chdr->hdrlen + 1) << 3) + sizeof(struct ipv6hdr));
+	skb_push(skb, chdr_len);
 	skb_reset_network_header(skb);
 	skb_mac_header_rebuild(skb);
 	skb_set_transport_header(skb, sizeof(struct ipv6hdr));
-- 
2.53.0