CVE-2026-53260
Full vulnerability record from the local CVENow database.
Description
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>
View original CVE JSON
{
"id": "CVE-2026-53260",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"published": "2026-06-25T09:16:43.993",
"lastModified": "2026-10-19T20:19:13.070",
"vulnStatus": "Modified",
"cveTags": [],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\ntcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().\n\nsyzbot reported a weird reqsk->rsk_refcnt underflow in\n__inet_csk_reqsk_queue_drop().\n\nThe captured reqsk_put() in __inet_csk_reqsk_queue_drop()\nis called only when it successfully removes reqsk from ehash.\n\nMoreover, reqsk_timer_handler() calls another reqsk_put()\nafter that.\n\nThis indicates that the reqsk was missing both refcnts for\nehash and the timer itself.\n\nSince all the syzbot reports had PREEMPT_RT enabled, the only\npossible scenario is that reqsk_queue_hash_req() is preempted\nafter mod_timer() and before refcount_set(), and then the timer\ntriggered after 1s aborts the reqsk due to its listener's close().\n\nLet's wrap mod_timer() and refcount_set() with\npreempt_disable_nested() and preempt_enable_nested().\n\nNote that inet_ehash_insert() holds the normal spin_lock()\n(mutex in PREEMPT_RT), so it must be called outside of\npreempt_disable_nested(), but this is fine.\n\nThe lookup path just ignores 0 sk_refcnt entries in ehash\nand tries to create another reqsk, but this will fail at\ninet_ehash_insert().\n\n[0]:\nrefcount_t: underflow; use-after-free.\nWARNING: lib/refcount.c:28 at refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28, CPU#0: ktimers/0/16\nModules linked in:\nCPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)}\nTainted: [L]=SOFTLOCKUP\nHardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026\nRIP: 0010:refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28\nCode: 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\nRSP: 0000:ffffc90000157948 EFLAGS: 00010246\nRAX: ffffffff84a1301b RBX: 0000000000000003 RCX: ffff88801ca98000\nRDX: 0000000000000100 RSI: 0000000000000000 RDI: ffffffff8f72ae00\nRBP: ffffffff99ae3b01 R08: ffff88801ca98000 R09: 0000000000000005\nR10: 0000000000000100 R11: 0000000000000004 R12: ffff8880425ef568\nR13: ffff8880425ef4f8 R14: ffff8880425ef578 R15: 0000000000000000\nFS: 0000000000000000(0000) GS:ffff888126386000(0000) knlGS:0000000000000000\nCS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033\nCR2: 00007f7b46710e9c CR3: 000000000dbb6000 CR4: 00000000003526f0\nCall Trace:\n <TASK>\n __refcount_sub_and_test include/linux/refcount.h:400 [inline]\n __refcount_dec_and_test include/linux/refcount.h:432 [inline]\n refcount_dec_and_test include/linux/refcount.h:450 [inline]\n reqsk_put include/net/request_sock.h:136 [inline]\n __inet_csk_reqsk_queue_drop+0x3ce/0x440 net/ipv4/inet_connection_sock.c:1007\n reqsk_timer_handler+0x651/0xdf0 net/ipv4/inet_connection_sock.c:1137\n call_timer_fn+0x192/0x5e0 kernel/time/timer.c:1748\n expire_timers kernel/time/timer.c:1799 [inline]\n __run_timers kernel/time/timer.c:2374 [inline]\n __run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2386\n run_timer_base kernel/time/timer.c:2395 [inline]\n run_timer_softirq+0x67/0x170 kernel/time/timer.c:2403\n handle_softirqs+0x1de/0x6d0 kernel/softirq.c:622\n __do_softirq kernel/softirq.c:656 [inline]\n run_ktimerd+0x69/0x100 kernel/softirq.c:1151\n smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160\n kthread+0x388/0x470 kernel/kthread.c:436\n ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158\n ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245\n </TASK>"
}
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"baseScore": 9.8,
"baseSeverity": "CRITICAL",
"attackVector": "NETWORK",
"attackComplexity": "LOW",
"privilegesRequired": "NONE",
"userInteraction": "NONE",
"scope": "UNCHANGED",
"confidentialityImpact": "HIGH",
"integrityImpact": "HIGH",
"availabilityImpact": "HIGH"
},
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"id": "CVE-2026-53260",
"options": [
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"description": [
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