CVE-2021-47126 in Linux
Summary
by MITRE • 03/15/2024
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: __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 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 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---
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Analysis
by VulDB Data Team • 06/21/2025
The vulnerability identified as CVE-2021-47126 resides within the Linux kernel's IPv6 routing subsystem, specifically in the function fib6_nh_flush_exceptions located in net/ipv6/route.c. This issue manifests as a KASAN (Kernel Address Sanitizer) slab-out-of-bounds read error, indicating that the kernel attempts to access memory outside the allocated bounds of a slab object. The flaw occurs during the processing of IPv6 routing table entries and exception handling, where the kernel fails to properly validate array indices when accessing exception buckets associated with next-hop structures. The reported out-of-bounds read happens at address ffff8880145c78f8 with a read size of 8 bytes, suggesting a potential dereference of a pointer beyond its valid memory range.
The technical root cause of this vulnerability lies in improper bounds checking within the fib6_nh_get_excptn_bucket function, which is invoked by fib6_nh_flush_exceptions. When the kernel processes routing exceptions for IPv6 next-hop structures, it fails to validate the index used to access exception bucket arrays. This condition can be triggered through specific network operations involving IPv6 routing table modifications, particularly when dealing with exception entries in the forwarding information base. The vulnerability is classified under CWE-129 as an "Improper Validation of Array Index" and represents a classic buffer overflow scenario where the kernel reads beyond allocated memory boundaries. The execution path leads through RCU (Read-Copy-Update) mechanisms and lockdep subsystems, indicating that the flaw may be exploitable in contexts involving concurrent access to routing table structures.
The operational impact of CVE-2021-47126 extends beyond simple memory corruption, as it can lead to system instability, denial of service, or potentially privilege escalation depending on exploitation context. While the immediate consequence is a kernel memory access violation that typically results in system crashes or hangs, the underlying flaw could be leveraged by attackers to gain unauthorized access to kernel memory spaces. This vulnerability affects all Linux kernel versions that include the problematic code path, particularly those using IPv6 routing functionality with exception handling. The flaw is particularly concerning in network-facing systems where IPv6 routing table modifications are frequent, such as routers, firewalls, and network infrastructure devices. The vulnerability's exploitation potential aligns with ATT&CK technique T1068 by leveraging local privilege escalation opportunities through kernel memory corruption, and T1499 for potential system instability or denial of service conditions.
Mitigation strategies for this vulnerability include applying the official kernel patch that corrects the bounds checking in the fib6_nh_get_excptn_bucket function, ensuring that array indices are properly validated before memory access occurs. System administrators should prioritize updating their Linux kernel installations to versions containing the fix, particularly in production environments handling IPv6 traffic. Additionally, monitoring for kernel memory access violations and implementing proper input validation for routing table operations can help detect potential exploitation attempts. The fix typically involves adding proper boundary checks to ensure that exception bucket indices remain within valid array limits before any memory access operations are performed. Organizations should also consider implementing network segmentation and access controls to limit exposure to potentially malicious IPv6 routing table modifications, as the vulnerability can be triggered through legitimate network operations that modify routing entries.