CVE-2021-47106: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2021-47106

In the Linux kernel, the following vulnerability has been resolved: netfilter: nf_tables: fix use-after-free in nft_set_catchall_destroy() We need to use list_for_each_entry_safe() iterator because we can not access @catchall after kfree_rcu() call. syzbot reported: BUG: KASAN: use-after-free in nft_set_catchall_destroy net/netfilter/nf_tables_api.c:4486 [inline] BUG: KASAN: use-after-free in nft_set_destroy net/netfilter/nf_tables_api.c:4504 [inline] BUG: KASAN: use-after-free in nft_set_destroy+0x3fd/0x4f0 net/netfilter/nf_tables_api.c:4493 Read of size 8 at addr ffff8880716e5b80 by task syz-executor.3/8871 CPU: 1 PID: 8871 Comm: syz-executor.3 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/0x2ed mm/kasan/report.c:247 __kasan_report mm/kasan/report.c:433 [inline] kasan_report.cold+0x83/0xdf mm/kasan/report.c:450 nft_set_catchall_destroy net/netfilter/nf_tables_api.c:4486 [inline] nft_set_destroy net/netfilter/nf_tables_api.c:4504 [inline] nft_set_destroy+0x3fd/0x4f0 net/netfilter/nf_tables_api.c:4493 __nft_release_table+0x79f/0xcd0 net/netfilter/nf_tables_api.c:9626 nft_rcv_nl_event+0x4f8/0x670 net/netfilter/nf_tables_api.c:9688 notifier_call_chain+0xb5/0x200 kernel/notifier.c:83 blocking_notifier_call_chain kernel/notifier.c:318 [inline] blocking_notifier_call_chain+0x67/0x90 kernel/notifier.c:306 netlink_release+0xcb6/0x1dd0 net/netlink/af_netlink.c:788 __sock_release+0xcd/0x280 net/socket.c:649 sock_close+0x18/0x20 net/socket.c:1314 __fput+0x286/0x9f0 fs/file_table.c:280 task_work_run+0xdd/0x1a0 kernel/task_work.c:164 tracehook_notify_resume include/linux/tracehook.h:189 [inline] exit_to_user_mode_loop kernel/entry/common.c:175 [inline] exit_to_user_mode_prepare+0x27e/0x290 kernel/entry/common.c:207 __syscall_exit_to_user_mode_work kernel/entry/common.c:289 [inline] syscall_exit_to_user_mode+0x19/0x60 kernel/entry/common.c:300 do_syscall_64+0x42/0xb0 arch/x86/entry/common.c:86 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x7f75fbf28adb Code: 0f 05 48 3d 00 f0 ff ff 77 45 c3 0f 1f 40 00 48 83 ec 18 89 7c 24 0c e8 63 fc ff ff 8b 7c 24 0c 41 89 c0 b8 03 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 35 44 89 c7 89 44 24 0c e8 a1 fc ff ff 8b 44 RSP: 002b:00007ffd8da7ec10 EFLAGS: 00000293 ORIG_RAX: 0000000000000003 RAX: 0000000000000000 RBX: 0000000000000004 RCX: 00007f75fbf28adb RDX: 00007f75fc08e828 RSI: ffffffffffffffff RDI: 0000000000000003 RBP: 00007f75fc08a960 R08: 0000000000000000 R09: 00007f75fc08e830 R10: 00007ffd8da7ed10 R11: 0000000000000293 R12: 00000000002067c3 R13: 00007ffd8da7ed10 R14: 00007f75fc088f60 R15: 0000000000000032 </TASK> Allocated by task 8886: 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_kmalloc mm/kasan/common.c:513 [inline] ____kasan_kmalloc mm/kasan/common.c:472 [inline] __kasan_kmalloc+0xa6/0xd0 mm/kasan/common.c:522 kasan_kmalloc include/linux/kasan.h:269 [inline] kmem_cache_alloc_trace+0x1ea/0x4a0 mm/slab.c:3575 kmalloc include/linux/slab.h:590 [inline] nft_setelem_catchall_insert net/netfilter/nf_tables_api.c:5544 [inline] nft_setelem_insert net/netfilter/nf_tables_api.c:5562 [inline] nft_add_set_elem+0x232e/0x2f40 net/netfilter/nf_tables_api.c:5936 nf_tables_newsetelem+0x6ff/0xbb0 net/netfilter/nf_tables_api.c:6032 nfnetlink_rcv_batch+0x1710/0x25f0 net/netfilter/nfnetlink.c:513 nfnetlink_rcv_skb_batch net/netfilter/nfnetlink.c:634 [inline] nfnetlink_rcv+0x3af/0x420 net/netfilter/nfnetlink.c:652 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/ ---truncated---

AI Analysis

Technical Summary

CVE-2021-47106 is a use-after-free vulnerability identified in the Linux kernel's netfilter subsystem, specifically within the nftables API implementation. The vulnerability arises in the function nft_set_catchall_destroy() due to improper handling of memory after a kfree_rcu() call. The root cause is the use of an unsafe iterator when destroying catchall elements in nftables sets, which leads to accessing freed memory. This was detected by the Kernel Address Sanitizer (KASAN) during fuzz testing by syzbot, which reported multiple use-after-free bugs in nft_set_catchall_destroy and nft_set_destroy functions. The issue occurs when the kernel attempts to destroy nftables sets and their catchall elements, leading to a read of freed memory, which can cause kernel crashes or potentially enable arbitrary code execution in kernel context. The vulnerability affects Linux kernel versions prior to the fix, which involves switching to the safe iterator list_for_each_entry_safe() to avoid accessing freed memory. The detailed kernel stack traces confirm the use-after-free condition and the involved kernel functions. No public exploits are known to exist in the wild as of the publication date. The vulnerability is critical because it affects the Linux kernel's packet filtering and firewalling subsystem, which is widely used in servers, cloud infrastructure, and network devices. Exploitation could lead to denial of service or privilege escalation.

Potential Impact

For European organizations, this vulnerability poses a significant risk especially for those relying on Linux-based servers, cloud platforms, and network infrastructure that utilize nftables for firewall and packet filtering. Exploitation could result in kernel crashes causing denial of service, impacting availability of critical services. More severely, attackers might leverage this use-after-free flaw to execute arbitrary code with kernel privileges, leading to full system compromise, data breaches, or lateral movement within networks. Given the widespread adoption of Linux in European data centers, telecommunications, and government infrastructure, the impact could be substantial. Organizations in sectors such as finance, healthcare, and critical infrastructure that depend on Linux security controls may face increased risk of disruption or data loss. The lack of known exploits currently reduces immediate threat but the vulnerability's nature and kernel-level impact make it a high-priority patching candidate to prevent future exploitation attempts.

Mitigation Recommendations

1. Immediate patching: Apply the official Linux kernel patches that fix the use-after-free in nft_set_catchall_destroy by adopting the safe iterator list_for_each_entry_safe(). Monitor Linux vendor advisories and update kernels promptly. 2. Kernel version management: Maintain an inventory of Linux kernel versions in use and prioritize upgrades for those running affected versions. 3. Network segmentation: Limit exposure of vulnerable Linux hosts by segmenting networks and restricting access to management interfaces that could trigger nftables operations. 4. Monitoring and detection: Deploy kernel-level monitoring tools and anomaly detection to identify unusual nftables activity or kernel crashes indicative of exploitation attempts. 5. Harden nftables usage: Review and minimize nftables rulesets to reduce complexity and attack surface, and avoid unnecessary dynamic set modifications that could trigger the vulnerable code paths. 6. Use of security modules: Employ Linux Security Modules (LSMs) like SELinux or AppArmor to restrict kernel module loading and limit damage from potential kernel exploits. 7. Incident response readiness: Prepare for potential exploitation by having forensic and recovery procedures in place to quickly respond to kernel-level compromises.