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

cipso_v4_skbuff_delattr() shifts every IPv4 option after CIPSO left by
cipso_len, but leaves the cached srr, rr, ts and router_alert offsets at
their old locations.

With a valid eight-byte CIPSO option followed by a seven-byte RR option,
RR moves from offset 28 to 20 while opt->rr remains 28.  The stale offset
then points into packet payload, letting the sender control both the
length and bytes copied by __ip_options_echo().  Setting the length to
255 produced this condensed KASAN result:

  BUG: KASAN: stack-out-of-bounds in __ip_options_echo
  Write of size 255
  Call Trace:
   __ip_options_echo
   run_cipso_stale_offset [net_candidate_probe]

The destination holds 40 bytes of option data.  In an existing
uninstrumented x86-64 v7.2 build, __icmp_send() places that buffer
immediately before its stack canary, and the 255-byte copy also crosses
the saved return address.  A canary mismatch unconditionally panics;
no canary disclosure or control-flow exploit was demonstrated.

Mirror cipso_v4_delopt() and subtract cipso_len from each cached offset
that follows CIPSO.  cipso_len is the logical distance moved by the first
memmove(); hdr_len_delta includes padding, while the later header move and
network-header reset relocate the bytes and their base together.

The adjusted input leaves rr at 20 with length 7 and completes without a
sanitizer report.  Natural remote reachability requires a configured
SELinux/NetLabel gateway and a later option consumer; the retained test
invokes the real mutation and echo consumer directly because the available
rootfs has no SELinux policy or netlabelctl.

Fixes: 89aa3619d141 ("cipso: make cipso_v4_skbuff_delattr() fully remove the CIPSO options")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <redacted>
---
 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 a05aa075de1a5..135b57c2c4f9e 100644
--- a/net/ipv4/cipso_ipv4.c
+++ b/net/ipv4/cipso_ipv4.c

@@ -2272,6 +2272,14 @@ int cipso_v4_skbuff_delattr(struct sk_buff *skb)
 	new_hdr_len_actual = hdr_len_actual - cipso_len;
 	new_hdr_len = (new_hdr_len_actual + 3) & ~3;
 	hdr_len_delta = (iph->ihl << 2) - new_hdr_len;
+	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;
 
 	/* 1. shift any options after CIPSO to the left */
 	memmove(cipso_ptr, cipso_ptr + cipso_len,

base-commit: 99b43ede9e355ba35244cc9470bf1819774ce39d
-- 
2.55.0