CVE-2021-47126: Vulnerability in Linux Linux

Severity: mediumType: vulnerabilityCVE-2021-47126

In the Linux kernel, the following vulnerability has been resolved: ipv6: Fix KASAN: slab-out-of-bounds Read in fib6_nh_flush_exceptions Reported by syzbot: HEAD commit: 90c911ad Merge tag 'fixes' of git://git.kernel.org/pub/scm.. git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master dashboard link: https://syzkaller.appspot.com/bug?extid=123aa35098fd3c000eb7 compiler: Debian clang version 11.0.1-2 ================================================================== BUG: KASAN: slab-out-of-bounds in fib6_nh_get_excptn_bucket net/ipv6/route.c:1604 [inline] BUG: KASAN: slab-out-of-bounds in fib6_nh_flush_exceptions+0xbd/0x360 net/ipv6/route.c:1732 Read of size 8 at addr ffff8880145c78f8 by task syz-executor.4/17760 CPU: 0 PID: 17760 Comm: syz-executor.4 Not tainted 5.12.0-rc8-syzkaller #0 Call Trace: <IRQ> __dump_stack lib/dump_stack.c:79 [inline] dump_stack+0x202/0x31e lib/dump_stack.c:120 print_address_description+0x5f/0x3b0 mm/kasan/report.c:232 __kasan_report mm/kasan/report.c:399 [inline] kasan_report+0x15c/0x200 mm/kasan/report.c:416 fib6_nh_get_excptn_bucket net/ipv6/route.c:1604 [inline] fib6_nh_flush_exceptions+0xbd/0x360 net/ipv6/route.c:1732 fib6_nh_release+0x9a/0x430 net/ipv6/route.c:3536 fib6_info_destroy_rcu+0xcb/0x1c0 net/ipv6/ip6_fib.c:174 rcu_do_batch kernel/rcu/tree.c:2559 [inline] rcu_core+0x8f6/0x1450 kernel/rcu/tree.c:2794 __do_softirq+0x372/0x7a6 kernel/softirq.c:345 invoke_softirq kernel/softirq.c:221 [inline] __irq_exit_rcu+0x22c/0x260 kernel/softirq.c:422 irq_exit_rcu+0x5/0x20 kernel/softirq.c:434 sysvec_apic_timer_interrupt+0x91/0xb0 arch/x86/kernel/apic/apic.c:1100 </IRQ> asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:632 RIP: 0010:lock_acquire+0x1f6/0x720 kernel/locking/lockdep.c:5515 Code: f6 84 24 a1 00 00 00 02 0f 85 8d 02 00 00 f7 c3 00 02 00 00 49 bd 00 00 00 00 00 fc ff df 74 01 fb 48 c7 44 24 40 0e 36 e0 45 <4b> c7 44 3d 00 00 00 00 00 4b c7 44 3d 09 00 00 00 00 43 c7 44 3d RSP: 0018:ffffc90009e06560 EFLAGS: 00000206 RAX: 1ffff920013c0cc0 RBX: 0000000000000246 RCX: dffffc0000000000 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 RBP: ffffc90009e066e0 R08: dffffc0000000000 R09: fffffbfff1f992b1 R10: fffffbfff1f992b1 R11: 0000000000000000 R12: 0000000000000000 R13: dffffc0000000000 R14: 0000000000000000 R15: 1ffff920013c0cb4 rcu_lock_acquire+0x2a/0x30 include/linux/rcupdate.h:267 rcu_read_lock include/linux/rcupdate.h:656 [inline] ext4_get_group_info+0xea/0x340 fs/ext4/ext4.h:3231 ext4_mb_prefetch+0x123/0x5d0 fs/ext4/mballoc.c:2212 ext4_mb_regular_allocator+0x8a5/0x28f0 fs/ext4/mballoc.c:2379 ext4_mb_new_blocks+0xc6e/0x24f0 fs/ext4/mballoc.c:4982 ext4_ext_map_blocks+0x2be3/0x7210 fs/ext4/extents.c:4238 ext4_map_blocks+0xab3/0x1cb0 fs/ext4/inode.c:638 ext4_getblk+0x187/0x6c0 fs/ext4/inode.c:848 ext4_bread+0x2a/0x1c0 fs/ext4/inode.c:900 ext4_append+0x1a4/0x360 fs/ext4/namei.c:67 ext4_init_new_dir+0x337/0xa10 fs/ext4/namei.c:2768 ext4_mkdir+0x4b8/0xc00 fs/ext4/namei.c:2814 vfs_mkdir+0x45b/0x640 fs/namei.c:3819 ovl_do_mkdir fs/overlayfs/overlayfs.h:161 [inline] ovl_mkdir_real+0x53/0x1a0 fs/overlayfs/dir.c:146 ovl_create_real+0x280/0x490 fs/overlayfs/dir.c:193 ovl_workdir_create+0x425/0x600 fs/overlayfs/super.c:788 ovl_make_workdir+0xed/0x1140 fs/overlayfs/super.c:1355 ovl_get_workdir fs/overlayfs/super.c:1492 [inline] ovl_fill_super+0x39ee/0x5370 fs/overlayfs/super.c:2035 mount_nodev+0x52/0xe0 fs/super.c:1413 legacy_get_tree+0xea/0x180 fs/fs_context.c:592 vfs_get_tree+0x86/0x270 fs/super.c:1497 do_new_mount fs/namespace.c:2903 [inline] path_mount+0x196f/0x2be0 fs/namespace.c:3233 do_mount fs/namespace.c:3246 [inline] __do_sys_mount fs/namespace.c:3454 [inline] __se_sys_mount+0x2f9/0x3b0 fs/namespace.c:3431 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x4665f9 Code: ff ff c3 66 2e 0f 1f 84 ---truncated---

AI Analysis

Technical Summary

CVE-2021-47126 is a medium-severity vulnerability in the Linux kernel related to IPv6 networking, specifically involving a slab-out-of-bounds read in the fib6_nh_flush_exceptions function within the IPv6 routing code. The issue was detected and reported by syzbot, an automated kernel fuzzing tool, and involves a memory safety bug categorized as CWE-125 (Out-of-bounds Read). The vulnerability arises from improper bounds checking when handling fib6_nh (IPv6 forwarding information base next-hop) exception buckets, leading to a read of memory beyond the allocated slab cache. This can cause kernel crashes or instability due to invalid memory access. The detailed kernel stack trace shows the fault occurs during the flushing of IPv6 next-hop exceptions, which is part of the kernel's routing table management. The vulnerability affects Linux kernel versions prior to the patch commit referenced, and it requires local privileges with low complexity to exploit (AV:L/AC:L/PR:L/UI:N). The CVSS v3.1 score is 5.5, reflecting a medium impact primarily on availability (kernel crash or denial of service), with no direct impact on confidentiality or integrity. No known exploits in the wild have been reported. The vulnerability does not require user interaction but does require at least low-level privileges, such as a local user or process with some access to networking or kernel interfaces. The issue was fixed in a recent kernel update by improving bounds checking and memory handling in the IPv6 routing code.

Potential Impact

For European organizations, this vulnerability poses a risk mainly to systems running vulnerable Linux kernel versions with IPv6 enabled, which is common in enterprise and cloud environments. Exploitation could lead to denial of service conditions by crashing the kernel or causing instability, potentially disrupting critical services, network infrastructure, or cloud workloads. Given the widespread use of Linux in servers, network appliances, and embedded devices across Europe, the impact could be significant in environments where uptime and availability are critical, such as telecommunications, finance, healthcare, and government sectors. However, since the vulnerability requires local privileges and does not allow privilege escalation or data compromise, the risk of direct data breach or system takeover is low. The vulnerability could be leveraged by attackers who have already gained limited access to escalate denial of service attacks, impacting service continuity. Organizations relying on IPv6 routing or complex network configurations should be particularly vigilant. The absence of known exploits reduces immediate risk but does not eliminate the need for timely patching.

Mitigation Recommendations

European organizations should prioritize updating Linux kernel versions to the patched releases that address CVE-2021-47126. Specifically, kernel updates from the official Linux distributions or vendors should be applied promptly. Network administrators should audit systems to identify those running vulnerable kernel versions with IPv6 enabled and schedule maintenance windows for patch deployment. Additionally, organizations should implement strict access controls to limit local user privileges, reducing the risk of exploitation by unprivileged users. Employing kernel hardening techniques such as Kernel Address Sanitizer (KASAN) in testing environments can help detect similar issues proactively. Monitoring system logs for kernel oops or crashes related to IPv6 routing may help detect attempted exploitation. For critical infrastructure, consider isolating or segmenting systems to limit the impact of potential denial of service. Finally, maintain up-to-date backups and incident response plans to recover quickly from any service disruptions caused by exploitation attempts.