CVE-2025-21959 in Linux
Summary
by MITRE • 04/01/2025
In the Linux kernel, the following vulnerability has been resolved:
netfilter: nf_conncount: Fully initialize struct nf_conncount_tuple in insert_tree()
Since commit b36e4523d4d5 ("netfilter: nf_conncount: fix garbage collection confirm race"), `cpu` and `jiffies32` were introduced to the struct nf_conncount_tuple.
The commit made nf_conncount_add() initialize `conn->cpu` and `conn->jiffies32` when allocating the struct. In contrast, count_tree() was not changed to initialize them.
By commit 34848d5c896e ("netfilter: nf_conncount: Split insert and traversal"), count_tree() was split and the relevant allocation code now resides in insert_tree(). Initialize `conn->cpu` and `conn->jiffies32` in insert_tree().
BUG: KMSAN: uninit-value in find_or_evict net/netfilter/nf_conncount.c:117 [inline]
BUG: KMSAN: uninit-value in __nf_conncount_add+0xd9c/0x2850 net/netfilter/nf_conncount.c:143 find_or_evict net/netfilter/nf_conncount.c:117 [inline]
__nf_conncount_add+0xd9c/0x2850 net/netfilter/nf_conncount.c:143 count_tree net/netfilter/nf_conncount.c:438 [inline]
nf_conncount_count+0x82f/0x1e80 net/netfilter/nf_conncount.c:521 connlimit_mt+0x7f6/0xbd0 net/netfilter/xt_connlimit.c:72 __nft_match_eval net/netfilter/nft_compat.c:403 [inline]
nft_match_eval+0x1a5/0x300 net/netfilter/nft_compat.c:433 expr_call_ops_eval net/netfilter/nf_tables_core.c:240 [inline]
nft_do_chain+0x426/0x2290 net/netfilter/nf_tables_core.c:288 nft_do_chain_ipv4+0x1a5/0x230 net/netfilter/nft_chain_filter.c:23 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626 nf_hook_slow_list+0x24d/0x860 net/netfilter/core.c:663 NF_HOOK_LIST include/linux/netfilter.h:350 [inline]
ip_sublist_rcv+0x17b7/0x17f0 net/ipv4/ip_input.c:633 ip_list_rcv+0x9ef/0xa40 net/ipv4/ip_input.c:669 __netif_receive_skb_list_ptype net/core/dev.c:5936 [inline]
__netif_receive_skb_list_core+0x15c5/0x1670 net/core/dev.c:5983 __netif_receive_skb_list net/core/dev.c:6035 [inline]
netif_receive_skb_list_internal+0x1085/0x1700 net/core/dev.c:6126 netif_receive_skb_list+0x5a/0x460 net/core/dev.c:6178 xdp_recv_frames net/bpf/test_run.c:280 [inline]
xdp_test_run_batch net/bpf/test_run.c:361 [inline]
bpf_test_run_xdp_live+0x2e86/0x3480 net/bpf/test_run.c:390 bpf_prog_test_run_xdp+0xf1d/0x1ae0 net/bpf/test_run.c:1316 bpf_prog_test_run+0x5e5/0xa30 kernel/bpf/syscall.c:4407 __sys_bpf+0x6aa/0xd90 kernel/bpf/syscall.c:5813 __do_sys_bpf kernel/bpf/syscall.c:5902 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5900 [inline]
__ia32_sys_bpf+0xa0/0xe0 kernel/bpf/syscall.c:5900 ia32_sys_call+0x394d/0x4180 arch/x86/include/generated/asm/syscalls_32.h:358 do_syscall_32_irqs_on arch/x86/entry/common.c:165 [inline]
__do_fast_syscall_32+0xb0/0x110 arch/x86/entry/common.c:387 do_fast_syscall_32+0x38/0x80 arch/x86/entry/common.c:412 do_SYSENTER_32+0x1f/0x30 arch/x86/entry/common.c:450 entry_SYSENTER_compat_after_hwframe+0x84/0x8e
Uninit was created at: slab_post_alloc_hook mm/slub.c:4121 [inline]
slab_alloc_node mm/slub.c:4164 [inline]
kmem_cache_alloc_noprof+0x915/0xe10 mm/slub.c:4171 insert_tree net/netfilter/nf_conncount.c:372 [inline]
count_tree net/netfilter/nf_conncount.c:450 [inline]
nf_conncount_count+0x1415/0x1e80 net/netfilter/nf_conncount.c:521 connlimit_mt+0x7f6/0xbd0 net/netfilter/xt_connlimit.c:72 __nft_match_eval net/netfilter/nft_compat.c:403 [inline]
nft_match_eval+0x1a5/0x300 net/netfilter/nft_compat.c:433 expr_call_ops_eval net/netfilter/nf_tables_core.c:240 [inline]
nft_do_chain+0x426/0x2290 net/netfilter/nf_tables_core.c:288 nft_do_chain_ipv4+0x1a5/0x230 net/netfilter/nft_chain_filter.c:23 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626 nf_hook_slow_list+0x24d/0x860 net/netfilter/core.c:663 NF_HOOK_LIST include/linux/netfilter.h:350 [inline]
ip_sublist_rcv+0x17b7/0x17f0 net/ipv4/ip_input.c:633 ip_list_rcv+0x9ef/0xa40 net/ip ---truncated---
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Analysis
by VulDB Data Team • 02/01/2026
The vulnerability CVE-2025-21959 resides within the Linux kernel's netfilter subsystem, specifically in the nf_conncount module responsible for connection tracking and limiting. This flaw manifests as an uninitialized memory access in the `struct nf_conncount_tuple` data structure, which is used to track network connections. The issue emerged following two key commits that modified how connection tracking data is handled. The first commit, b36e4523d4d5, introduced `cpu` and `jiffies32` fields to the `nf_conncount_tuple` structure to address a garbage collection race condition. Subsequently, commit 34848d5c896e split the `count_tree()` function, relocating allocation logic into `insert_tree()`. However, the initialization of the newly added fields was not properly propagated to `insert_tree()`, leaving `cpu` and `jiffies32` uninitialized when connections are inserted into the tracking tree.
The technical flaw directly impacts the integrity of connection tracking operations by causing undefined behavior when the uninitialized fields are accessed. Memory Sanitizer (KMSAN) reports an uninit-value error in the `find_or_evict` function, indicating that code paths involving `nf_conncount_count` and `connlimit_mt` attempt to read memory that has not been properly initialized. This vulnerability can be triggered through netfilter operations, particularly when processing packets via the `xt_connlimit` module, which enforces connection limits on network traffic. The execution flow shows that the bug propagates from packet reception through the netfilter hook chain, ultimately reaching the problematic `insert_tree()` function where uninitialized data is read.
The operational impact of this vulnerability extends to the reliability and correctness of connection tracking functionality in Linux systems. An attacker could potentially exploit this flaw to cause system instability or manipulate connection tracking behavior, leading to denial of service or bypass of connection limit rules. The vulnerability is particularly concerning in environments where netfilter is heavily utilized for traffic control, firewalling, or network security enforcement. According to CWE-457, this represents an uninitialized variable issue, while the ATT&CK framework would categorize this under privilege escalation or defense evasion tactics if exploited to manipulate kernel behavior. The vulnerability affects systems running kernel versions that include the problematic commits, with the risk being highest in configurations using connection tracking limits or packet filtering rules.
Mitigation of this vulnerability requires updating to a patched kernel version that ensures proper initialization of the `cpu` and `jiffies32` fields within `insert_tree()`. Administrators should prioritize kernel updates, particularly in production environments where netfilter modules are actively used for traffic control. System administrators can also consider disabling unused netfilter modules or connection tracking features if the risk is deemed high. Monitoring for KMSAN or similar memory error reports can help identify systems affected by this issue. Additionally, implementing proper kernel hardening measures such as stack canaries and address space layout randomization can reduce the exploitability of such uninitialized memory issues, though they do not directly address the root cause. The fix implemented in the patch ensures that all code paths allocating `nf_conncount_tuple` structures properly initialize all fields, thereby preventing the undefined behavior that leads to system instability or potential privilege escalation.