EUVD-2026-39211
GHSA-cmhx-q3fp-jhf6
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Triggered by remote SYN traffic (AV:N, PR:N) but only on PREEMPT_RT kernels and requires winning a tight preemption race (AC:H); impact is primarily kernel crash (A:H) with limited corruption (I:L) and no confidentiality.
Primary rating from Vendor (416baaa9-dc9f-4396-8d5f-8c081fb06d67).
CVSS VectorVendor: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Lifecycle Timeline
4DescriptionCVE.org
In the Linux kernel, the following vulnerability has been resolved:
tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().
syzbot reported a weird reqsk->rsk_refcnt underflow in __inet_csk_reqsk_queue_drop().
The captured reqsk_put() in __inet_csk_reqsk_queue_drop() is called only when it successfully removes reqsk from ehash.
Moreover, reqsk_timer_handler() calls another reqsk_put() after that.
This indicates that the reqsk was missing both refcnts for ehash and the timer itself.
Since all the syzbot reports had PREEMPT_RT enabled, the only possible scenario is that reqsk_queue_hash_req() is preempted after mod_timer() and before refcount_set(), and then the timer triggered after 1s aborts the reqsk due to its listener's close().
Let's wrap mod_timer() and refcount_set() with preempt_disable_nested() and preempt_enable_nested().
Note that inet_ehash_insert() holds the normal spin_lock() (mutex in PREEMPT_RT), so it must be called outside of preempt_disable_nested(), but this is fine.
The lookup path just ignores 0 sk_refcnt entries in ehash and tries to create another reqsk, but this will fail at inet_ehash_insert().
[0]: refcount_t: underflow; use-after-free. WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28, CPU#0: ktimers/0/16 Modules linked in: CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)} Tainted: [L]=SOFTLOCKUP Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026 RIP: 0010:refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28 Code: e4 7d d1 0a 67 48 0f b9 3a eb 4a e8 38 3d 23 fd 48 8d 3d e1 7d d1 0a 67 48 0f b9 3a eb 37 e8 25 3d 23 fd 48 8d 3d de 7d d1 0a <67> 48 0f b9 3a eb 24 e8 12 3d 23 fd 48 8d 3d db 7d d1 0a 67 48 0f RSP: 0000:ffffc90000157948 EFLAGS: 00010246 RAX: ffffffff84a1301b RBX: 0000000000000003 RCX: ffff88801ca98000 RDX: 0000000000000100 RSI: 0000000000000000 RDI: ffffffff8f72ae00 RBP: ffffffff99ae3b01 R08: ffff88801ca98000 R09: 0000000000000005 R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880425ef568 R13: ffff8880425ef4f8 R14: ffff8880425ef578 R15: 0000000000000000 FS: 0000000000000000(0000) GS:ffff888126386000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f7b46710e9c CR3: 000000000dbb6000 CR4: 00000000003526f0 Call Trace: <TASK> __refcount_sub_and_test include/linux/refcount.h:400 [inline] __refcount_dec_and_test include/linux/refcount.h:432 [inline] refcount_dec_and_test include/linux/refcount.h:450 [inline] reqsk_put include/net/request_sock.h:136 [inline] __inet_csk_reqsk_queue_drop+0x3ce/0x440 net/ipv4/inet_connection_sock.c:1007 reqsk_timer_handler+0x651/0xdf0 net/ipv4/inet_connection_sock.c:1137 call_timer_fn+0x192/0x5e0 kernel/time/timer.c:1748 expire_timers kernel/time/timer.c:1799 [inline] __run_timers kernel/time/timer.c:2374 [inline] __run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2386 run_timer_base kernel/time/timer.c:2395 [inline] run_timer_softirq+0x67/0x170 kernel/time/timer.c:2403 handle_softirqs+0x1de/0x6d0 kernel/softirq.c:622 __do_softirq kernel/softirq.c:656 [inline] run_ktimerd+0x69/0x100 kernel/softirq.c:1151 smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160 kthread+0x388/0x470 kernel/kthread.c:436 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245 </TASK>
AnalysisAI
Denial of service and potential memory corruption in the Linux kernel TCP stack arises from a refcount underflow / use-after-free in reqsk_queue_hash_req(), affecting kernels built with PREEMPT_RT (real-time preemption). On affected systems a request socket (reqsk) can lose both its ehash and timer reference counts when reqsk_queue_hash_req() is preempted between mod_timer() and refcount_set(), letting reqsk_timer_handler() drop the object twice and trigger a use-after-free flagged by refcount_warn_saturate. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires the target to be running a Linux kernel compiled with PREEMPT_RT (full real-time preemption) - on all syzbot reports this config was present and it is the only scenario in which the bug manifests. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The signals conflict sharply. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | On a PREEMPT_RT kernel, an attacker (or adversarial workload) drives high-volume inbound TCP SYN traffic against a listening socket while that listener is repeatedly closed, racing to preempt reqsk_queue_hash_req() between mod_timer() and refcount_set(). When the race is won, the 1-second request-socket timer fires and double-drops the reqsk, causing a refcount underflow and use-after-free that crashes the kernel or corrupts memory. … |
| Remediation | Upstream fix available (PR/commit); the change wraps mod_timer() and refcount_set() with preempt_disable_nested()/preempt_enable_nested(). … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours: Conduct inventory of Linux systems to identify any running PREEMPT_RT-enabled kernels (verify with uname -a output containing 'PREEMPT_RT'); if none found, document and close. …
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Same weakness CWE-416 – Use After Free
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External POC / Exploit Code
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