[PATCH net v4] cipso: fix optlen corruption in cipso_v4_delopt()

[Qihang <q.h.hack.winter@gmail.com>]

cipso_v4_delopt() recomputes the option length by walking the option
buffer after the CIPSO option has been moved out.  The walk then reads
the stale bytes at the tail of the buffer as option data, and a crafted
setsockopt(IP_OPTIONS) buffer can make it treat one of them as an option
length, increasing opt->opt.optlen beyond the valid option data.

The corrupted length stays on the socket and is later used as the copy
length by getsockopt(IP_OPTIONS) and by the udp/raw/ping sendmsg paths,
where it exceeds the size of the buffers they copy into.

Compute the new length from the intact options before the move and
subtract the CIPSO length, and fill the padding with IPOPT_END.

Assisted-by: LLM
Fixes: 014ab19a69c32 ("selinux: Set socket NetLabel based on connection endpoint")
Cc: stable@vger.kernel.org
Signed-off-by: Qihang <q.h.hack.winter@gmail.com>
---
v4:
 - rebased onto net (fcc00b15cb21a)
 - reworded the impact paragraph only, no functional change
 - first public posting (v1-v3 were reviewed privately)

 net/ipv4/cipso_ipv4.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
index 1aacbbeffbc64..e65a3f29d2caf 100644
--- a/net/ipv4/cipso_ipv4.c
+++ b/net/ipv4/cipso_ipv4.c
@@ -2013,6 +2013,10 @@ static int cipso_v4_delopt(struct ip_options_rcu __rcu **opt_ptr)
 		cipso_ptr = &opt->opt.__data[cipso_off];
 		cipso_len = cipso_ptr[1];
 
+		optlen_new = cipso_v4_get_actual_opt_len(opt->opt.__data,
+							 opt->opt.optlen);
+		optlen_new -= cipso_len;
+
 		if (opt->opt.srr > opt->opt.cipso)
 			opt->opt.srr -= cipso_len;
 		if (opt->opt.rr > opt->opt.cipso)
@@ -2026,11 +2030,12 @@ static int cipso_v4_delopt(struct ip_options_rcu __rcu **opt_ptr)
 		memmove(cipso_ptr, cipso_ptr + cipso_len,
 			opt->opt.optlen - cipso_off - cipso_len);
 
-		optlen_new = cipso_v4_get_actual_opt_len(opt->opt.__data,
-							 opt->opt.optlen);
 		hdr_delta = opt->opt.optlen;
 		opt->opt.optlen = (optlen_new + 3) & ~3;
 		hdr_delta -= opt->opt.optlen;
+
+		memset(&opt->opt.__data[optlen_new], IPOPT_END,
+		       opt->opt.optlen - optlen_new);
 	} else {
 		/* only the cipso option was present on the socket so we can
 		 * remove the entire option struct */

base-commit: fcc00b15cb21a6de76826e5012b2c758505f5e65
-- 
2.54.0 (Apple Git-157)