CVE-2025-40186 - Vulnerability Details

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:
<IRQ>
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)
</IRQ>

No CVSS v4.0

Attack Vector Local

Attack Complexity High

Privileges Required Low

Scope Unchanged

Confidentiality Impact High

Integrity Impact High

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

The EPSS score is 0.00024.

Key SSVC decision points have not yet been added.

Vendors	Products
Linux	Linux Kernel

No data.

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Vendors	Products
Linux	Linux Kernel

Advisories

No advisories yet.

Fixes

Solution

No solution given by the vendor.

Workaround

No workaround given by the vendor.

References

Link	Providers
https://git.kernel.org/stable/c/2e7cbbbe3d61c63606994b7ff73c72537afe2e1c
https://git.kernel.org/stable/c/422c1c173c39bbbae1e0eaaf8aefe40b2596233b
https://git.kernel.org/stable/c/643a94b0cf767325e953591c212be2eb826b9d7f
https://git.kernel.org/stable/c/64dc47a13aa3d9daf7cec29b44dca8e22a6aea15
https://git.kernel.org/stable/c/c11ace909e873118295e9eb22dc8c58b0b50eb32
https://git.kernel.org/stable/c/e359b742eac1eac75cff4e38ee2e8cea492acd9b
https://git.kernel.org/stable/c/eb85ad5f23268d64b037bfb545cbcba3752f90c7
https://git.kernel.org/stable/c/ff6a8883f96a5bc74241ce5b3d431a6dcfa2124d
https://lore.kernel.org/linux-cve-announce/2025111244-CVE-2025-40186-b204@gregkh/T
https://nvd.nist.gov/vuln/detail/CVE-2025-40186
https://www.cve.org/CVERecord?id=CVE-2025-40186

History

Thu, 13 Nov 2025 12:15:00 +0000

Type	Values Removed	Values Added
References		https://lore.kernel.org/linux-cve-announce/2025111244-CVE-2025-40186-b204@gregkh/T https://nvd.nist.gov/vuln/detail/CVE-2025-40186 https://www.cve.org/CVERecord?id=CVE-2025-40186
Metrics	threat_severity `None`	cvssV3_1 `{'score': 7.0, 'vector': 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H'}` threat_severity `Important`

Thu, 13 Nov 2025 10:00:00 +0000

Type	Values Removed	Values Added
First Time appeared		Linux Linux linux Kernel
Vendors & Products		Linux Linux linux Kernel

Wed, 12 Nov 2025 22:15:00 +0000

Type	Values Removed	Values Added
Description		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: <IRQ> 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) </IRQ>
Title		tcp: Don't call reqsk_fastopen_remove() in tcp_conn_request().
References		https://git.kernel.org/stable/c/2e7cbbbe3d61c63606994b7ff73c72537afe2e1c https://git.kernel.org/stable/c/422c1c173c39bbbae1e0eaaf8aefe40b2596233b https://git.kernel.org/stable/c/643a94b0cf767325e953591c212be2eb826b9d7f https://git.kernel.org/stable/c/64dc47a13aa3d9daf7cec29b44dca8e22a6aea15 https://git.kernel.org/stable/c/c11ace909e873118295e9eb22dc8c58b0b50eb32 https://git.kernel.org/stable/c/e359b742eac1eac75cff4e38ee2e8cea492acd9b https://git.kernel.org/stable/c/eb85ad5f23268d64b037bfb545cbcba3752f90c7 https://git.kernel.org/stable/c/ff6a8883f96a5bc74241ce5b3d431a6dcfa2124d

MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2025-11-12T21:56:29.033Z

Updated: 2025-11-12T21:56:29.033Z

Reserved: 2025-04-16T07:20:57.177Z

Link: CVE-2025-40186

Vulnrichment

No data.

NVD

Status : Awaiting Analysis

Published: 2025-11-12T22:15:45.443

Modified: 2025-11-14T16:42:30.503

Link: CVE-2025-40186

Redhat

Severity : Important

Publid Date: 2025-11-12T00:00:00Z

Links: CVE-2025-40186 - Bugzilla

OpenCVE Enrichment

Updated: 2025-11-13T09:52:07Z

Metrics

Attack Vector Local

Attack Complexity High

Privileges Required Low

Scope Unchanged

Confidentiality Impact High

Integrity Impact High

Availability Impact High

User Interaction None

Affected Vendors & Products

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