🇰🇷 한글 설명
Linux 커널의 TCP RTO(재전송 타임아웃) 처리 코드에서 skb(소켓 버퍼) NULL 포인터를 검사하지 않아 발생하는 취약점. tcp_rto_delta_us() 함수가 NULL 체크 없이 메모리 접근을 시도하여 커널 패닉을 유발합니다.
💡 쉬운 설명
Linux가 느린 네트워크 연결에서 데이터 재전송을 시도할 때 존재하지 않는 메모리에 접근해 시스템이 충돌합니다. 네트워크 트래픽으로 악용 가능하며 서비스 중단을 초래할 수 있습니다.
🇺🇸 원문 (English)
In the Linux kernel, the following vulnerability has been resolved:
tcp: check skb is non-NULL in tcp_rto_delta_us()
We have some machines running stock Ubuntu 20.04.6 which is their 5.4.0-174-generic
kernel that are running ceph and recently hit a null ptr dereference in
tcp_rearm_rto(). Initially hitting it from the TLP path, but then later we also
saw it getting hit from the RACK case as well. Here are examples of the oops
messages we saw in each of those cases:
Jul 26 15:05:02 rx [11061395.780353] BUG: kernel NULL pointer dereference, address: 0000000000000020
Jul 26 15:05:02 rx [11061395.787572] #PF: supervisor read access in kernel mode
Jul 26 15:05:02 rx [11061395.792971] #PF: error_code(0x0000) - not-present page
Jul 26 15:05:02 rx [11061395.798362] PGD 0 P4D 0
Jul 26 15:05:02 rx [11061395.801164] Oops: 0000 [#1] SMP NOPTI
Jul 26 15:05:02 rx [11061395.805091] CPU: 0 PID: 9180 Comm: msgr-worker-1 Tainted: G W 5.4.0-174-generic #193-Ubuntu
Jul 26 15:05:02 rx [11061395.814996] Hardware name: Supermicro SMC 2x26 os-gen8 64C NVME-Y 256G/H12SSW-NTR, BIOS 2.5.V1.2U.NVMe.UEFI 05/09/2023
Jul 26 15:05:02 rx [11061395.825952] RIP: 0010:tcp_rearm_rto+0xe4/0x160
Jul 26 15:05:02 rx [11061395.830656] Code: 87 ca 04 00 00 00 5b 41 5c 41 5d 5d c3 c3 49 8b bc 24 40 06 00 00 eb 8d 48 bb cf f7 53 e3 a5 9b c4 20 4c 89 ef e8 0c fe 0e 00 <48> 8b 78 20 48 c1 ef 03 48 89 f8 41 8b bc 24 80 04 00 00 48 f7 e3
Jul 26 15:05:02 rx [11061395.849665] RSP: 0018:ffffb75d40003e08 EFLAGS: 00010246
Jul 26 15:05:02 rx [11061395.855149] RAX: 0000000000000000 RBX: 20c49ba5e353f7cf RCX: 0000000000000000
Jul 26 15:05:02 rx [11061395.862542] RDX: 0000000062177c30 RSI: 000000000000231c RDI: ffff9874ad283a60
Jul 26 15:05:02 rx [11061395.869933] RBP: ffffb75d40003e20 R08: 0000000000000000 R09: ffff987605e20aa8
Jul 26 15:05:02 rx [11061395.877318] R10: ffffb75d40003f00 R11: ffffb75d4460f740 R12: ffff9874ad283900
Jul 26 15:05:02 rx [11061395.884710] R13: ffff9874ad283a60 R14: ffff9874ad283980 R15: ffff9874ad283d30
Jul 26 15:05:02 rx [11061395.892095] FS: 00007f1ef4a2e700(0000) GS:ffff987605e00000(0000) knlGS:0000000000000000
Jul 26 15:05:02 rx [11061395.900438] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Jul 26 15:05:02 rx [11061395.906435] CR2: 0000000000000020 CR3: 0000003e450ba003 CR4: 0000000000760ef0
Jul 26 15:05:02 rx [11061395.913822] PKRU: 55555554
Jul 26 15:05:02 rx [11061395.916786] Call Trace:
Jul 26 15:05:02 rx [11061395.919488]
Jul 26 15:05:02 rx [11061395.921765] ? show_regs.cold+0x1a/0x1f
Jul 26 15:05:02 rx [11061395.925859] ? __die+0x90/0xd9
Jul 26 15:05:02 rx [11061395.929169] ? no_context+0x196/0x380
Jul 26 15:05:02 rx [11061395.933088] ? ip6_protocol_deliver_rcu+0x4e0/0x4e0
Jul 26 15:05:02 rx [11061395.938216] ? ip6_sublist_rcv_finish+0x3d/0x50
Jul 26 15:05:02 rx [11061395.943000] ? __bad_area_nosemaphore+0x50/0x1a0
Jul 26 15:05:02 rx [11061395.947873] ? bad_area_nosemaphore+0x16/0x20
Jul 26 15:05:02 rx [11061395.952486] ? do_user_addr_fault+0x267/0x450
Jul 26 15:05:02 rx [11061395.957104] ? ipv6_list_rcv+0x112/0x140
Jul 26 15:05:02 rx [11061395.961279] ? __do_page_fault+0x58/0x90
Jul 26 15:05:02 rx [11061395.965458] ? do_page_fault+0x2c/0xe0
Jul 26 15:05:02 rx [11061395.969465] ? page_fault+0x34/0x40
Jul 26 15:05:02 rx [11061395.973217] ? tcp_rearm_rto+0xe4/0x160
Jul 26 15:05:02 rx [11061395.977313] ? tcp_rearm_rto+0xe4/0x160
Jul 26 15:05:02 rx [11061395.981408] tcp_send_loss_probe+0x10b/0x220
Jul 26 15:05:02 rx [11061395.985937] tcp_write_timer_handler+0x1b4/0x240
Jul 26 15:05:02 rx [11061395.990809] tcp_write_timer+0x9e/0xe0
Jul 26 15:05:02 rx [11061395.994814] ? tcp_write_timer_handler+0x240/0x240
Jul 26 15:05:02 rx [11061395.999866] call_timer_fn+0x32/0x130
Jul 26 15:05:02 rx [11061396.003782] __run_timers.part.0+0x180/0x280
Jul 26 15:05:02 rx [11061396.008309] ? recalibrate_cpu_khz+0x10/0x10
Jul 26 15:05:02 rx [11061396.012841] ? native_x2apic_icr_write+0x30/0x30
Jul 26 15:05:02 rx [11061396.017718] ? lapic_next_even
---truncated---
📍 영향받는 시스템
Linux kernel 5.4.0-174-generic 포함 Ubuntu 20.04.6 및 동일 계열 커널 버전 (5.4.x 대)
⚔️ 공격 벡터
TLP(Tail Loss Probe) 또는 RACK 재전송 경로를 통해 NULL 포인터 dereference를 유발하는 네트워크 패킷 트래픽
🛡️ 대응 방법
Linux 커널을 tcp_rto_delta_us() 함수에 skb NULL 체크를 추가한 패치 버전으로 업데이트
🔧 검증 코드
현재 커널 버전이 5.4.0-174 이전 버전이면 취약 버전이고, 패치된 최신 버전으로 업데이트되었는지 확인합니다.
uname -r && cat /proc/version | grep -oE '5\.4\.[0-9]+-[0-9]+'