CVE-2021-47557 in Linux
Summary
by MITRE • 05/24/2024
In the Linux kernel, the following vulnerability has been resolved:
net/sched: sch_ets: don't peek at classes beyond 'nbands'
when the number of DRR classes decreases, the round-robin active list can contain elements that have already been freed in ets_qdisc_change(). As a consequence, it's possible to see a NULL dereference crash, caused by the attempt to call cl->qdisc->ops->peek(cl->qdisc) when cl->qdisc is NULL:
BUG: kernel NULL pointer dereference, address: 0000000000000018 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 1 PID: 910 Comm: mausezahn Not tainted 5.16.0-rc1+ #475 Hardware name: Red Hat KVM, BIOS 1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014 RIP: 0010:ets_qdisc_dequeue+0x129/0x2c0 [sch_ets]
Code: c5 01 41 39 ad e4 02 00 00 0f 87 18 ff ff ff 49 8b 85 c0 02 00 00 49 39 c4 0f 84 ba 00 00 00 49 8b ad c0 02 00 00 48 8b 7d 10 8b 47 18 48 8b 40 38 0f ae e8 ff d0 48 89 c3 48 85 c0 0f 84 9d RSP: 0000:ffffbb36c0b5fdd8 EFLAGS: 00010287 RAX: ffff956678efed30 RBX: 0000000000000000 RCX: 0000000000000000 RDX: 0000000000000002 RSI: ffffffff9b938dc9 RDI: 0000000000000000 RBP: ffff956678efed30 R08: e2f3207fe360129c R09: 0000000000000000 R10: 0000000000000001 R11: 0000000000000001 R12: ffff956678efeac0 R13: ffff956678efe800 R14: ffff956611545000 R15: ffff95667ac8f100 FS: 00007f2aa9120740(0000) GS:ffff95667b800000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000018 CR3: 000000011070c000 CR4: 0000000000350ee0 Call Trace: qdisc_peek_dequeued+0x29/0x70 [sch_ets]
tbf_dequeue+0x22/0x260 [sch_tbf]
__qdisc_run+0x7f/0x630 net_tx_action+0x290/0x4c0 __do_softirq+0xee/0x4f8 irq_exit_rcu+0xf4/0x130 sysvec_apic_timer_interrupt+0x52/0xc0 asm_sysvec_apic_timer_interrupt+0x12/0x20 RIP: 0033:0x7f2aa7fc9ad4 Code: b9 ff ff 48 8b 54 24 18 48 83 c4 08 48 89 ee 48 89 df 5b 5d e9 ed fc ff ff 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 f3 0f 1e fa 48 83 ec 10 48 8b 05 10 64 33 00 48 8b 00 48 85 c0 0f 85 84 00 RSP: 002b:00007ffe5d33fab8 EFLAGS: 00000202 RAX: 0000000000000002 RBX: 0000561f72c31460 RCX: 0000561f72c31720 RDX: 0000000000000002 RSI: 0000561f72c31722 RDI: 0000561f72c31720 RBP: 000000000000002a R08: 00007ffe5d33fa40 R09: 0000000000000014 R10: 0000000000000000 R11: 0000000000000246 R12: 0000561f7187e380 R13: 0000000000000000 R14: 0000000000000000 R15: 0000561f72c31460 Modules linked in: sch_ets sch_tbf dummy rfkill iTCO_wdt intel_rapl_msr iTCO_vendor_support intel_rapl_common joydev virtio_balloon lpc_ich i2c_i801 i2c_smbus pcspkr ip_tables xfs libcrc32c crct10dif_pclmul crc32_pclmul crc32c_intel ahci libahci ghash_clmulni_intel serio_raw libata virtio_blk virtio_console virtio_net net_failover failover sunrpc dm_mirror dm_region_hash dm_log dm_mod CR2: 0000000000000018
Ensuring that 'alist' was never zeroed [1] was not sufficient, we need to
remove from the active list those elements that are no more SP nor DRR.
[1] https://lore.kernel.org/netdev/60d274838bf09777f0371253416e8af71360bc08.1633609148.git.dcaratti@redhat.com/
v3: fix race between ets_qdisc_change() and ets_qdisc_dequeue() delisting DRR classes beyond 'nbands' in ets_qdisc_change() with the qdisc lock acquired, thanks to Cong Wang.
v2: when a NULL qdisc is found in the DRR active list, try to dequeue skb from the next list item.
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Analysis
by VulDB Data Team • 07/12/2025
The vulnerability CVE-2021-47557 resides within the Linux kernel's traffic control subsystem, specifically in the Enhanced Transmission Selection (ETS) queuing discipline implementation. This flaw manifests as a NULL pointer dereference during packet dequeuing operations, stemming from improper handling of DRR (Deficit Round Robin) classes when their count decreases. The issue occurs in the sch_ets module where the round-robin active list may retain references to classes that have already been freed during queuing discipline changes, creating a scenario where code attempts to access a NULL qdisc pointer through the peek operation.
The technical root cause involves a race condition between the ets_qdisc_change() function and the ets_qdisc_dequeue() function, where elements are not properly removed from the active list when the number of DRR classes decreases. When the number of bands in the ETS configuration reduces, the active list can contain stale references to previously freed classes. The system attempts to call cl->qdisc->ops->peek(cl->qdisc) where cl->qdisc is NULL, leading to the kernel NULL pointer dereference crash. This memory access violation occurs at address 0x0000000000000018, corresponding to the qdisc pointer field within the class structure, as evidenced by the kernel oops trace showing execution within ets_qdisc_dequeue function.
This vulnerability directly maps to CWE-476, which describes NULL pointer dereference conditions, and aligns with ATT&CK technique T1547.001 for privilege escalation through kernel exploitation. The operational impact includes potential system crashes and denial of service conditions, particularly affecting network-intensive workloads that utilize ETS queuing disciplines. The vulnerability is exploitable in environments where dynamic queuing discipline changes occur, such as in virtualized networking environments or systems using advanced traffic control configurations. The fix implemented addresses the race condition by ensuring proper cleanup of DRR classes beyond the configured nbands limit during queuing discipline changes, with the qdisc lock properly acquired to prevent concurrent access issues. The mitigation strategy involves updating to kernel versions that include the patched sch_ets implementation, which properly removes stale entries from the active list and handles NULL qdisc pointers gracefully during dequeuing operations.