CVE-2021-47103: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2021-47103

In the Linux kernel, the following vulnerability has been resolved: inet: fully convert sk->sk_rx_dst to RCU rules syzbot reported various issues around early demux, one being included in this changelog [1] sk->sk_rx_dst is using RCU protection without clearly documenting it. And following sequences in tcp_v4_do_rcv()/tcp_v6_do_rcv() are not following standard RCU rules. [a] dst_release(dst); [b] sk->sk_rx_dst = NULL; They look wrong because a delete operation of RCU protected pointer is supposed to clear the pointer before the call_rcu()/synchronize_rcu() guarding actual memory freeing. In some cases indeed, dst could be freed before [b] is done. We could cheat by clearing sk_rx_dst before calling dst_release(), but this seems the right time to stick to standard RCU annotations and debugging facilities. [1] BUG: KASAN: use-after-free in dst_check include/net/dst.h:470 [inline] BUG: KASAN: use-after-free in tcp_v4_early_demux+0x95b/0x960 net/ipv4/tcp_ipv4.c:1792 Read of size 2 at addr ffff88807f1cb73a by task syz-executor.5/9204 CPU: 0 PID: 9204 Comm: syz-executor.5 Not tainted 5.16.0-rc5-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: <TASK> __dump_stack lib/dump_stack.c:88 [inline] dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106 print_address_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247 __kasan_report mm/kasan/report.c:433 [inline] kasan_report.cold+0x83/0xdf mm/kasan/report.c:450 dst_check include/net/dst.h:470 [inline] tcp_v4_early_demux+0x95b/0x960 net/ipv4/tcp_ipv4.c:1792 ip_rcv_finish_core.constprop.0+0x15de/0x1e80 net/ipv4/ip_input.c:340 ip_list_rcv_finish.constprop.0+0x1b2/0x6e0 net/ipv4/ip_input.c:583 ip_sublist_rcv net/ipv4/ip_input.c:609 [inline] ip_list_rcv+0x34e/0x490 net/ipv4/ip_input.c:644 __netif_receive_skb_list_ptype net/core/dev.c:5508 [inline] __netif_receive_skb_list_core+0x549/0x8e0 net/core/dev.c:5556 __netif_receive_skb_list net/core/dev.c:5608 [inline] netif_receive_skb_list_internal+0x75e/0xd80 net/core/dev.c:5699 gro_normal_list net/core/dev.c:5853 [inline] gro_normal_list net/core/dev.c:5849 [inline] napi_complete_done+0x1f1/0x880 net/core/dev.c:6590 virtqueue_napi_complete drivers/net/virtio_net.c:339 [inline] virtnet_poll+0xca2/0x11b0 drivers/net/virtio_net.c:1557 __napi_poll+0xaf/0x440 net/core/dev.c:7023 napi_poll net/core/dev.c:7090 [inline] net_rx_action+0x801/0xb40 net/core/dev.c:7177 __do_softirq+0x29b/0x9c2 kernel/softirq.c:558 invoke_softirq kernel/softirq.c:432 [inline] __irq_exit_rcu+0x123/0x180 kernel/softirq.c:637 irq_exit_rcu+0x5/0x20 kernel/softirq.c:649 common_interrupt+0x52/0xc0 arch/x86/kernel/irq.c:240 asm_common_interrupt+0x1e/0x40 arch/x86/include/asm/idtentry.h:629 RIP: 0033:0x7f5e972bfd57 Code: 39 d1 73 14 0f 1f 80 00 00 00 00 48 8b 50 f8 48 83 e8 08 48 39 ca 77 f3 48 39 c3 73 3e 48 89 13 48 8b 50 f8 48 89 38 49 8b 0e <48> 8b 3e 48 83 c3 08 48 83 c6 08 eb bc 48 39 d1 72 9e 48 39 d0 73 RSP: 002b:00007fff8a413210 EFLAGS: 00000283 RAX: 00007f5e97108990 RBX: 00007f5e97108338 RCX: ffffffff81d3aa45 RDX: ffffffff81d3aa45 RSI: 00007f5e97108340 RDI: ffffffff81d3aa45 RBP: 00007f5e97107eb8 R08: 00007f5e97108d88 R09: 0000000093c2e8d9 R10: 0000000000000000 R11: 0000000000000000 R12: 00007f5e97107eb0 R13: 00007f5e97108338 R14: 00007f5e97107ea8 R15: 0000000000000019 </TASK> Allocated by task 13: kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38 kasan_set_track mm/kasan/common.c:46 [inline] set_alloc_info mm/kasan/common.c:434 [inline] __kasan_slab_alloc+0x90/0xc0 mm/kasan/common.c:467 kasan_slab_alloc include/linux/kasan.h:259 [inline] slab_post_alloc_hook mm/slab.h:519 [inline] slab_alloc_node mm/slub.c:3234 [inline] slab_alloc mm/slub.c:3242 [inline] kmem_cache_alloc+0x202/0x3a0 mm/slub.c:3247 dst_alloc+0x146/0x1f0 net/core/dst.c:92 rt_dst_alloc+0x73/0x430 net/ipv4/route.c:1613 ip_route_input_slow+0x1817/0x3a20 net/ipv4/route.c:234 ---truncated---

AI Analysis

Technical Summary

CVE-2021-47103 is a vulnerability identified in the Linux kernel's networking stack, specifically related to the handling of the sk->sk_rx_dst pointer within the inet subsystem. The issue arises from improper use of Read-Copy-Update (RCU) synchronization mechanisms when managing this pointer in the tcp_v4_do_rcv() and tcp_v6_do_rcv() functions. The vulnerability was initially reported by syzbot, a kernel fuzzing tool, which detected use-after-free bugs during early demultiplexing of TCP packets. The root cause is that the sk_rx_dst pointer, which is protected by RCU, is not cleared before the associated memory is freed via dst_release(), violating standard RCU rules. This can lead to a use-after-free condition where the kernel attempts to access freed memory, potentially causing kernel crashes or memory corruption. The detailed kernel stack trace and KASAN (Kernel Address Sanitizer) reports confirm the presence of these use-after-free errors. The vulnerability affects Linux kernel versions identified by the commit hash 41063e9dd11956f2d285e12e4342e1d232ba0ea2 and similar revisions. No public exploits are currently known in the wild, and no CVSS score has been assigned yet. However, the issue is critical in nature due to its potential to cause system instability and possible escalation of privileges if exploited. The fix involves properly adhering to RCU rules by clearing the sk_rx_dst pointer before freeing the associated destination structure, ensuring safe memory management and preventing use-after-free conditions.

Potential Impact

For European organizations, this vulnerability poses a significant risk to any systems running affected Linux kernel versions, particularly those handling network traffic at scale such as servers, network appliances, and cloud infrastructure. Exploitation could lead to kernel crashes resulting in denial of service (DoS), impacting availability of critical services. In worst-case scenarios, memory corruption could be leveraged by attackers to execute arbitrary code with kernel privileges, compromising confidentiality and integrity of data. Given the widespread use of Linux in European data centers, telecommunications, and government infrastructure, the impact could be substantial. Organizations relying on Linux-based virtual machines or containers in cloud environments are also at risk. The vulnerability's exploitation does not require user interaction but does require the attacker to send crafted network packets to the vulnerable system, making it a realistic threat vector in exposed network environments. The absence of known exploits currently reduces immediate risk, but the technical nature of the flaw and its presence in core networking code warrant urgent attention to prevent future exploitation.

Mitigation Recommendations

European organizations should prioritize updating their Linux kernel to versions where this vulnerability is patched. Since the vulnerability relates to kernel internals, applying vendor-provided kernel updates or mainline kernel patches is the most effective mitigation. For environments where immediate patching is not feasible, organizations should implement network-level controls to restrict exposure of vulnerable systems, such as firewall rules limiting inbound TCP traffic to trusted sources only. Monitoring kernel logs for unusual crashes or KASAN reports can help detect exploitation attempts. Additionally, organizations should audit their Linux kernel versions across all infrastructure, including cloud instances and embedded devices, to identify vulnerable systems. Employing kernel hardening techniques such as Kernel Page Table Isolation (KPTI) and enabling kernel lockdown features can reduce exploitation impact. Finally, maintaining robust incident response capabilities and regular backups will mitigate damage in case of successful exploitation.