--- id: CVE-2025-40186 title: 'tcp: Don''t call reqsk_fastopen_remove() in tcp_conn_request().' summary: |- In the Linux kernel, the following vulnerability has been resolved: tcp: Don't call reqsk_fastopen_remove() in tcp_conn_request(). syzbot reported the splat below in tcp_conn_request(). [0] If a listener is close()d while a TFO socket… severity: high cvss: 8.1 cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H' cvssSource: cna vendor: Linux product: Linux affected: - >- Linux >= 7ec092a91ff351dcde89c23e795b73a328274db6 < e359b742eac1eac75cff4e38ee2e8cea492acd9b - >- Linux >= a4378dedd6e07e62f2fccb17d78c9665718763d0 < ff6a8883f96a5bc74241ce5b3d431a6dcfa2124d - >- Linux >= 33a4fdf0b4a25f8ce65380c3b0136b407ca57609 < eb85ad5f23268d64b037bfb545cbcba3752f90c7 - >- Linux >= 17d699727577814198d744d6afe54735c6b54c99 < 643a94b0cf767325e953591c212be2eb826b9d7f - >- Linux >= dfd06131107e7b699ef1e2a24ed2f7d17c917753 < 422c1c173c39bbbae1e0eaaf8aefe40b2596233b - >- Linux >= fa4749c065644af4db496b338452a69a3e5147d9 < c11ace909e873118295e9eb22dc8c58b0b50eb32 - >- Linux >= 45c8a6cc2bcd780e634a6ba8e46bffbdf1fc5c01 < 64dc47a13aa3d9daf7cec29b44dca8e22a6aea15 - >- Linux >= 45c8a6cc2bcd780e634a6ba8e46bffbdf1fc5c01 < 2e7cbbbe3d61c63606994b7ff73c72537afe2e1c - Linux ae313d14b45eca7a6bb29cb9bf396d977e7d28fb - Linux >= 5.4.300 < 5.4.301 - Linux >= 5.10.245 < 5.10.246 - Linux >= 5.15.194 < 5.15.195 - Linux >= 6.1.154 < 6.1.157 - Linux >= 6.6.108 < 6.6.113 - Linux >= 6.12.49 < 6.12.54 - Linux >= 6.16.9 < 6.17 - Linux 6.17 ssvc: exploitation: none automatable: 'no' technicalImpact: total timestamp: '2026-10-15T21:57:46.472825Z' published: '2025-11-12' updated: '2026-10-15' sourceUpdated: '2026-10-15T22:02:39.982Z' source: CVEORG sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-40186' references: - url: 'https://git.kernel.org/stable/c/e359b742eac1eac75cff4e38ee2e8cea492acd9b' - url: 'https://git.kernel.org/stable/c/ff6a8883f96a5bc74241ce5b3d431a6dcfa2124d' - url: 'https://git.kernel.org/stable/c/eb85ad5f23268d64b037bfb545cbcba3752f90c7' - url: 'https://git.kernel.org/stable/c/643a94b0cf767325e953591c212be2eb826b9d7f' - url: 'https://git.kernel.org/stable/c/422c1c173c39bbbae1e0eaaf8aefe40b2596233b' - url: 'https://git.kernel.org/stable/c/c11ace909e873118295e9eb22dc8c58b0b50eb32' - url: 'https://git.kernel.org/stable/c/64dc47a13aa3d9daf7cec29b44dca8e22a6aea15' - url: 'https://git.kernel.org/stable/c/2e7cbbbe3d61c63606994b7ff73c72537afe2e1c' tags: - cve.org epss: 0.0058 epssPercentile: 0.46385 ingestedAt: '2026-10-15T22:48:25.550Z' --- ## Overview In the Linux kernel, the following vulnerability has been resolved: tcp: Don't call reqsk_fastopen_remove() in tcp_conn_request(). syzbot reported the splat below in tcp_conn_request(). [0] If a listener is close()d while a TFO socket is being processed in tcp_conn_request(), inet_csk_reqsk_queue_add() does not set reqsk->sk and calls inet_child_forget(), which calls tcp_disconnect() for the TFO socket. After the cited commit, tcp_disconnect() calls reqsk_fastopen_remove(), where reqsk_put() is called due to !reqsk->sk. Then, reqsk_fastopen_remove() in tcp_conn_request() decrements the last req->rsk_refcnt and frees reqsk, and __reqsk_free() at the drop_and_free label causes the refcount underflow for the listener and double-free of the reqsk. Let's remove reqsk_fastopen_remove() in tcp_conn_request(). Note that other callers make sure tp->fastopen_rsk is not NULL. [0]: refcount_t: underflow; use-after-free. WARNING: CPU: 12 PID: 5563 at lib/refcount.c:28 refcount_warn_saturate (lib/refcount.c:28) Modules linked in: CPU: 12 UID: 0 PID: 5563 Comm: syz-executor Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025 RIP: 0010:refcount_warn_saturate (lib/refcount.c:28) Code: ab e8 8e b4 98 ff 0f 0b c3 cc cc cc cc cc 80 3d a4 e4 d6 01 00 75 9c c6 05 9b e4 d6 01 01 48 c7 c7 e8 df fb ab e8 6a b4 98 ff <0f> 0b e9 03 5b 76 00 cc 80 3d 7d e4 d6 01 00 0f 85 74 ff ff ff c6 RSP: 0018:ffffa79fc0304a98 EFLAGS: 00010246 RAX: d83af4db1c6b3900 RBX: ffff9f65c7a69020 RCX: d83af4db1c6b3900 RDX: 0000000000000000 RSI: 00000000ffff7fff RDI: ffffffffac78a280 RBP: 000000009d781b60 R08: 0000000000007fff R09: ffffffffac6ca280 R10: 0000000000017ffd R11: 0000000000000004 R12: ffff9f65c7b4f100 R13: ffff9f65c7d23c00 R14: ffff9f65c7d26000 R15: ffff9f65c7a64ef8 FS: 00007f9f962176c0(0000) GS:ffff9f65fcf00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000200000000180 CR3: 000000000dbbe006 CR4: 0000000000372ef0 Call Trace: tcp_conn_request (./include/linux/refcount.h:400 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 ./include/net/sock.h:1965 ./include/net/request_sock.h:131 net/ipv4/tcp_input.c:7301) tcp_rcv_state_process (net/ipv4/tcp_input.c:6708) tcp_v6_do_rcv (net/ipv6/tcp_ipv6.c:1670) tcp_v6_rcv (net/ipv6/tcp_ipv6.c:1906) ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:438) ip6_input (net/ipv6/ip6_input.c:500) ipv6_rcv (net/ipv6/ip6_input.c:311) __netif_receive_skb (net/core/dev.c:6104) process_backlog (net/core/dev.c:6456) __napi_poll (net/core/dev.c:7506) net_rx_action (net/core/dev.c:7569 net/core/dev.c:7696) handle_softirqs (kernel/softirq.c:579) do_softirq (kernel/softirq.c:480) ## Affected - `Linux >= 7ec092a91ff351dcde89c23e795b73a328274db6 < e359b742eac1eac75cff4e38ee2e8cea492acd9b` - `Linux >= a4378dedd6e07e62f2fccb17d78c9665718763d0 < ff6a8883f96a5bc74241ce5b3d431a6dcfa2124d` - `Linux >= 33a4fdf0b4a25f8ce65380c3b0136b407ca57609 < eb85ad5f23268d64b037bfb545cbcba3752f90c7` - `Linux >= 17d699727577814198d744d6afe54735c6b54c99 < 643a94b0cf767325e953591c212be2eb826b9d7f` - `Linux >= dfd06131107e7b699ef1e2a24ed2f7d17c917753 < 422c1c173c39bbbae1e0eaaf8aefe40b2596233b` - `Linux >= fa4749c065644af4db496b338452a69a3e5147d9 < c11ace909e873118295e9eb22dc8c58b0b50eb32` - `Linux >= 45c8a6cc2bcd780e634a6ba8e46bffbdf1fc5c01 < 64dc47a13aa3d9daf7cec29b44dca8e22a6aea15` - `Linux >= 45c8a6cc2bcd780e634a6ba8e46bffbdf1fc5c01 < 2e7cbbbe3d61c63606994b7ff73c72537afe2e1c` - `Linux ae313d14b45eca7a6bb29cb9bf396d977e7d28fb` - `Linux >= 5.4.300 < 5.4.301` - `Linux >= 5.10.245 < 5.10.246` - `Linux >= 5.15.194 < 5.15.195` - `Linux >= 6.1.154 < 6.1.157` - `Linux >= 6.6.108 < 6.6.113` - `Linux >= 6.12.49 < 6.12.54` - `Linux >= 6.16.9 < 6.17` - `Linux 6.17` ## Remediation Refer to the linked advisories for vendor-supplied fixes and affected version ranges.