CVE-2021-47598 in Linux
Summary
by MITRE • 06/19/2024
In the Linux kernel, the following vulnerability has been resolved:
sch_cake: do not call cake_destroy() from cake_init()
qdiscs are not supposed to call their own destroy() method from init(), because core stack already does that.
syzbot was able to trigger use after free:
DEBUG_LOCKS_WARN_ON(lock->magic != lock) WARNING: CPU: 0 PID: 21902 at kernel/locking/mutex.c:586 __mutex_lock_common kernel/locking/mutex.c:586 [inline]
WARNING: CPU: 0 PID: 21902 at kernel/locking/mutex.c:586 __mutex_lock+0x9ec/0x12f0 kernel/locking/mutex.c:740 Modules linked in: CPU: 0 PID: 21902 Comm: syz-executor189 Not tainted 5.16.0-rc4-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 RIP: 0010:__mutex_lock_common kernel/locking/mutex.c:586 [inline]
RIP: 0010:__mutex_lock+0x9ec/0x12f0 kernel/locking/mutex.c:740 Code: 08 84 d2 0f 85 19 08 00 00 8b 05 97 38 4b 04 85 c0 0f 85 27 f7 ff ff 48 c7 c6 20 00 ac 89 48 c7 c7 a0 fe ab 89 e8 bf 76 ba ff 0b e9 0d f7 ff ff 48 8b 44 24 40 48 8d b8 c8 08 00 00 48 89 f8 RSP: 0018:ffffc9000627f290 EFLAGS: 00010282 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 RDX: ffff88802315d700 RSI: ffffffff815f1db8 RDI: fffff52000c4fe44 RBP: ffff88818f28e000 R08: 0000000000000000 R09: 0000000000000000 R10: ffffffff815ebb5e R11: 0000000000000000 R12: 0000000000000000 R13: dffffc0000000000 R14: ffffc9000627f458 R15: 0000000093c30000 FS: 0000555556abc400(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fda689c3303 CR3: 000000001cfbb000 CR4: 0000000000350ef0 Call Trace: tcf_chain0_head_change_cb_del+0x2e/0x3d0 net/sched/cls_api.c:810 tcf_block_put_ext net/sched/cls_api.c:1381 [inline]
tcf_block_put_ext net/sched/cls_api.c:1376 [inline]
tcf_block_put+0xbc/0x130 net/sched/cls_api.c:1394 cake_destroy+0x3f/0x80 net/sched/sch_cake.c:2695 qdisc_create.constprop.0+0x9da/0x10f0 net/sched/sch_api.c:1293 tc_modify_qdisc+0x4c5/0x1980 net/sched/sch_api.c:1660 rtnetlink_rcv_msg+0x413/0xb80 net/core/rtnetlink.c:5571 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2496 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1345 netlink_sendmsg+0x904/0xdf0 net/netlink/af_netlink.c:1921 sock_sendmsg_nosec net/socket.c:704 [inline]
sock_sendmsg+0xcf/0x120 net/socket.c:724 ____sys_sendmsg+0x6e8/0x810 net/socket.c:2409 ___sys_sendmsg+0xf3/0x170 net/socket.c:2463 __sys_sendmsg+0xe5/0x1b0 net/socket.c:2492 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x7f1bb06badb9 Code: Unable to access opcode bytes at RIP 0x7f1bb06bad8f. RSP: 002b:00007fff3012a658 EFLAGS: 00000246 ORIG_RAX: 000000000000002e RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f1bb06badb9 RDX: 0000000000000000 RSI: 00000000200007c0 RDI: 0000000000000003 RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000000000003 R10: 0000000000000003 R11: 0000000000000246 R12: 00007fff3012a688 R13: 00007fff3012a6a0 R14: 00007fff3012a6e0 R15: 00000000000013c2
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Analysis
by VulDB Data Team • 10/04/2025
The vulnerability CVE-2021-47598 resides within the Linux kernel's traffic control subsystem, specifically in the sch_cake qdisc implementation. This issue manifests as an improper initialization sequence where the cake_init() function directly calls cake_destroy(), a pattern that violates fundamental kernel design principles. The qdisc initialization process should not invoke its own destruction routine, as the kernel core stack already manages this lifecycle through dedicated mechanisms. This improper calling sequence creates a race condition and potential use-after-free scenario that can be exploited by malicious actors or automated testing frameworks like syzbot.
The technical flaw stems from a violation of the kernel's qdisc lifecycle management protocols. When sch_cake initializes, it attempts to destroy itself before completing initialization, leading to a situation where memory structures are prematurely deallocated while still being referenced. The kernel's locking subsystem detects this anomaly through the DEBUG_LOCKS_WARN_ON mechanism, specifically triggering on mutex lock validation failures. The stack trace reveals that the error originates from tcf_chain0_head_change_cb_del, which eventually calls cake_destroy, demonstrating how the improper initialization cascades through the traffic control API layers. This behavior creates a scenario where kernel memory is accessed after deallocation, resulting in undefined behavior and potential system instability.
The operational impact of this vulnerability extends beyond simple system crashes, as it can be leveraged for privilege escalation or denial-of-service attacks. The use-after-free condition allows an attacker to potentially corrupt kernel memory, leading to arbitrary code execution in kernel space. This represents a critical security risk since the kernel's traffic control subsystem is frequently used for network traffic management and quality of service enforcement. The vulnerability can be triggered through network configuration operations involving traffic control commands, making it accessible through various legitimate kernel interfaces. The syzbot automated testing framework specifically identified this issue, indicating that it can be reliably reproduced under controlled conditions, which suggests the vulnerability may be exploitable in real-world scenarios.
Mitigation strategies for CVE-2021-47598 focus on correcting the initialization sequence within the sch_cake qdisc implementation. The fix involves removing the direct call to cake_destroy() from cake_init(), ensuring that qdisc destruction occurs only through the proper kernel management pathways. This change aligns with the established kernel design patterns where qdiscs are initialized through standard APIs and destroyed through the core qdisc management system. System administrators should ensure their kernels are updated to versions containing the patched implementation, as the vulnerability affects the core networking stack. Additionally, monitoring for unusual network traffic control operations and implementing proper kernel hardening measures can help detect exploitation attempts. This vulnerability is classified under CWE-459, representing incomplete cleanup, and aligns with ATT&CK techniques involving privilege escalation through kernel exploits, making it a critical vulnerability requiring immediate attention.