CVE-2025-38181 in Linux
Summary
by MITRE • 07/04/2025
In the Linux kernel, the following vulnerability has been resolved:
calipso: Fix null-ptr-deref in calipso_req_{set,del}attr().
syzkaller reported a null-ptr-deref in sock_omalloc() while allocating a CALIPSO option. [0]
The NULL is of struct sock, which was fetched by sk_to_full_sk() in calipso_req_setattr().
Since commit a1a5344ddbe8 ("tcp: avoid two atomic ops for syncookies"), reqsk->rsk_listener could be NULL when SYN Cookie is returned to its client, as hinted by the leading SYN Cookie log.
Here are 3 options to fix the bug:
1) Return 0 in calipso_req_setattr() 2) Return an error in calipso_req_setattr() 3) Alaways set rsk_listener
1) is no go as it bypasses LSM, but 2) effectively disables SYN Cookie for CALIPSO. 3) is also no go as there have been many efforts to reduce atomic ops and make TCP robust against DDoS. See also commit 3b24d854cb35 ("tcp/dccp: do not touch listener sk_refcnt under synflood").
As of the blamed commit, SYN Cookie already did not need refcounting, and no one has stumbled on the bug for 9 years, so no CALIPSO user will care about SYN Cookie.
Let's return an error in calipso_req_setattr() and calipso_req_delattr() in the SYN Cookie case.
This can be reproduced by [1] on Fedora and now connect() of nc times out.
[0]:
TCP: request_sock_TCPv6: Possible SYN flooding on port [::]:20002. Sending cookies.
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] PREEMPT SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
CPU: 3 UID: 0 PID: 12262 Comm: syz.1.2611 Not tainted 6.14.0 #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 RIP: 0010:read_pnet include/net/net_namespace.h:406 [inline]
RIP: 0010:sock_net include/net/sock.h:655 [inline]
RIP: 0010:sock_kmalloc+0x35/0x170 net/core/sock.c:2806 Code: 89 d5 41 54 55 89 f5 53 48 89 fb e8 25 e3 c6 fd e8 f0 91 e3 00 48 8d 7b 30 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 26 01 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b RSP: 0018:ffff88811af89038 EFLAGS: 00010216 RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffff888105266400 RDX: 0000000000000006 RSI: ffff88800c890000 RDI: 0000000000000030 RBP: 0000000000000050 R08: 0000000000000000 R09: ffff88810526640e R10: ffffed1020a4cc81 R11: ffff88810526640f R12: 0000000000000000 R13: 0000000000000820 R14: ffff888105266400 R15: 0000000000000050 FS: 00007f0653a07640(0000) GS:ffff88811af80000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f863ba096f4 CR3: 00000000163c0005 CR4: 0000000000770ef0 PKRU: 80000000 Call Trace: <IRQ> ipv6_renew_options+0x279/0x950 net/ipv6/exthdrs.c:1288 calipso_req_setattr+0x181/0x340 net/ipv6/calipso.c:1204 calipso_req_setattr+0x56/0x80 net/netlabel/netlabel_calipso.c:597 netlbl_req_setattr+0x18a/0x440 net/netlabel/netlabel_kapi.c:1249 selinux_netlbl_inet_conn_request+0x1fb/0x320 security/selinux/netlabel.c:342 selinux_inet_conn_request+0x1eb/0x2c0 security/selinux/hooks.c:5551 security_inet_conn_request+0x50/0xa0 security/security.c:4945 tcp_v6_route_req+0x22c/0x550 net/ipv6/tcp_ipv6.c:825 tcp_conn_request+0xec8/0x2b70 net/ipv4/tcp_input.c:7275 tcp_v6_conn_request+0x1e3/0x440 net/ipv6/tcp_ipv6.c:1328 tcp_rcv_state_process+0xafa/0x52b0 net/ipv4/tcp_input.c:6781 tcp_v6_do_rcv+0x8a6/0x1a40 net/ipv6/tcp_ipv6.c:1667 tcp_v6_rcv+0x505e/0x5b50 net/ipv6/tcp_ipv6.c:1904 ip6_protocol_deliver_rcu+0x17c/0x1da0 net/ipv6/ip6_input.c:436 ip6_input_finish+0x103/0x180 net/ipv6/ip6_input.c:480 NF_HOOK include/linux/netfilter.h:314 [inline]
NF_HOOK include/linux/netfilter.h:308 [inline]
ip6_input+0x13c/0x6b0 net/ipv6/ip6_input.c:491 dst_input include/net/dst.h:469 [inline]
ip6_rcv_finish net/ipv6/ip6_input.c:79 [inline]
ip6_rcv_finish+0xb6/0x490 net/ipv6/ip6_input.c:69 NF_HOOK include/linux/netfilter.h:314 [inline]
NF_HOOK include/linux/netf ---truncated---
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Analysis
by VulDB Data Team • 12/19/2025
The vulnerability described in CVE-2025-38181 resides within the Linux kernel's implementation of the CALIPSO protocol, specifically in the functions calipso_req_setattr() and calipso_req_delattr(). This flaw manifests as a null pointer dereference occurring during the allocation of CALIPSO options, which is triggered when handling TCP SYN cookies. The issue arises from a change introduced in commit a1a5344ddbe8, which modified how SYN cookies are processed, leading to situations where reqsk->rsk_listener can be NULL. When the kernel attempts to access this NULL pointer through sk_to_full_sk() in calipso_req_setattr(), it results in a general protection fault and subsequent system crash. The vulnerability is particularly concerning because it can be exploited through network-based attacks that trigger SYN flooding conditions, potentially leading to denial of service. The root cause lies in the interaction between TCP SYN cookie handling and CALIPSO option processing, where the kernel does not properly account for cases where the listener socket reference is not set. This aligns with CWE-476, which describes null pointer dereference vulnerabilities, and maps to ATT&CK technique T1499.004 for network denial of service attacks. The fix implemented involves returning an error in both calipso_req_setattr() and calipso_req_delattr() when the SYN cookie path is detected, effectively preventing the null pointer dereference while maintaining system stability. This approach avoids bypassing security mechanisms and maintains the integrity of the TCP stack's SYN cookie functionality, though it does disable CALIPSO support in SYN cookie scenarios. The vulnerability has remained undetected for approximately nine years, indicating low impact in real-world deployments where CALIPSO is rarely used, but it still represents a critical kernel stability issue. The syzkaller fuzzer was instrumental in identifying this issue, demonstrating how automated testing can uncover subtle race conditions and edge cases in kernel code. The patch specifically targets the problematic code path by checking for NULL rsk_listener references and returning appropriate error codes rather than attempting to dereference the null pointer. This vulnerability highlights the complexity of maintaining backward compatibility while improving performance and security in kernel networking stacks, particularly when dealing with protocol extensions like CALIPSO that interact with core TCP handling mechanisms. The fix ensures that when SYN cookies are used, CALIPSO options are not processed, preventing the crash while maintaining the security benefits of SYN cookie protection against DDoS attacks. This approach aligns with the kernel's broader strategy of reducing atomic operations for performance and robustness, as indicated by related commits such as 3b24d854cb35 that emphasize avoiding unnecessary reference counting under SYN flood conditions. The vulnerability demonstrates how seemingly isolated changes in kernel networking code can have unexpected cascading effects when multiple subsystems interact, particularly in high-concurrency scenarios involving connection handling and security policy enforcement.