[PATCH net v2] cipso: adjust cached option offsets when removing CIPSO

[Daehyeon Ko <4ncienth@gmail.com>]

cipso_v4_skbuff_delattr() removes the CIPSO bytes but does not adjust
cached offsets for options that follow them.

For example, with a valid 10-byte CIPSO option followed by a seven-byte
RR option, parsing records rr at offset 30.  Removing CIPSO moves RR to
offset 20, while the cached offset remains 30.  Consumers such as
ip_forward_options() and __ip_options_echo() then access the wrong bytes;
the latter may interpret packet data as the option length and copy it into
fixed-size option storage.

Mirror cipso_v4_delopt() and subtract cipso_len from the srr, rr, ts and
router_alert offsets when they follow CIPSO.  cipso_len is the distance
the first memmove() shifts those options.  The later header move and
network-header reset relocate the bytes and their offset base together.

Fixes: 89aa3619d141 ("cipso: make cipso_v4_skbuff_delattr() fully remove the CIPSO options")
Cc: stable@vger.kernel.org
Reviewed-by: Ondrej Mosnáček <omosnacek@gmail.com>
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Changes in v2:
- Replace the parser-invalid 8-byte example with a valid 10-byte one.
- Move the offset updates beside the other option metadata updates.

Link: https://lore.kernel.org/netdev/20260930140400.2955466-1-4ncienth@gmail.com/

 net/ipv4/cipso_ipv4.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
index a05aa075de1a5b..1aacbbeffbc647 100644
--- a/net/ipv4/cipso_ipv4.c
+++ b/net/ipv4/cipso_ipv4.c
@@ -2287,6 +2287,14 @@ int cipso_v4_skbuff_delattr(struct sk_buff *skb)
 	       new_hdr_len - new_hdr_len_actual);
 
 	opt->optlen -= hdr_len_delta;
+	if (opt->srr > opt->cipso)
+		opt->srr -= cipso_len;
+	if (opt->rr > opt->cipso)
+		opt->rr -= cipso_len;
+	if (opt->ts > opt->cipso)
+		opt->ts -= cipso_len;
+	if (opt->router_alert > opt->cipso)
+		opt->router_alert -= cipso_len;
 	opt->cipso = 0;
 	opt->is_changed = 1;
 	if (hdr_len_delta != 0) {

base-commit: d5a007b9b457c915ab1a53227e8939e4018aa97a
-- 
2.55.0