CVE-2025-38472 (GCVE-0-2025-38472)
Vulnerability from cvelistv5
Published
2025-07-28 11:21
Modified
2026-08-05 12:02
Severity ?
9.8 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: netfilter: nf_conntrack: fix crash due to removal of uninitialised entry A crash in conntrack was reported while trying to unlink the conntrack entry from the hash bucket list: [exception RIP: __nf_ct_delete_from_lists+172] [..] #7 [ff539b5a2b043aa0] nf_ct_delete at ffffffffc124d421 [nf_conntrack] #8 [ff539b5a2b043ad0] nf_ct_gc_expired at ffffffffc124d999 [nf_conntrack] #9 [ff539b5a2b043ae0] __nf_conntrack_find_get at ffffffffc124efbc [nf_conntrack] [..] The nf_conn struct is marked as allocated from slab but appears to be in a partially initialised state: ct hlist pointer is garbage; looks like the ct hash value (hence crash). ct->status is equal to IPS_CONFIRMED|IPS_DYING, which is expected ct->timeout is 30000 (=30s), which is unexpected. Everything else looks like normal udp conntrack entry. If we ignore ct->status and pretend its 0, the entry matches those that are newly allocated but not yet inserted into the hash: - ct hlist pointers are overloaded and store/cache the raw tuple hash - ct->timeout matches the relative time expected for a new udp flow rather than the absolute 'jiffies' value. If it were not for the presence of IPS_CONFIRMED, __nf_conntrack_find_get() would have skipped the entry. Theory is that we did hit following race: cpu x cpu y cpu z found entry E found entry E E is expired <preemption> nf_ct_delete() return E to rcu slab init_conntrack E is re-inited, ct->status set to 0 reply tuplehash hnnode.pprev stores hash value. cpu y found E right before it was deleted on cpu x. E is now re-inited on cpu z. cpu y was preempted before checking for expiry and/or confirm bit. ->refcnt set to 1 E now owned by skb ->timeout set to 30000 If cpu y were to resume now, it would observe E as expired but would skip E due to missing CONFIRMED bit. nf_conntrack_confirm gets called sets: ct->status |= CONFIRMED This is wrong: E is not yet added to hashtable. cpu y resumes, it observes E as expired but CONFIRMED: <resumes> nf_ct_expired() -> yes (ct->timeout is 30s) confirmed bit set. cpu y will try to delete E from the hashtable: nf_ct_delete() -> set DYING bit __nf_ct_delete_from_lists Even this scenario doesn't guarantee a crash: cpu z still holds the table bucket lock(s) so y blocks: wait for spinlock held by z CONFIRMED is set but there is no guarantee ct will be added to hash: "chaintoolong" or "clash resolution" logic both skip the insert step. reply hnnode.pprev still stores the hash value. unlocks spinlock return NF_DROP <unblocks, then crashes on hlist_nulls_del_rcu pprev> In case CPU z does insert the entry into the hashtable, cpu y will unlink E again right away but no crash occurs. Without 'cpu y' race, 'garbage' hlist is of no consequence: ct refcnt remains at 1, eventually skb will be free'd and E gets destroyed via: nf_conntrack_put -> nf_conntrack_destroy -> nf_ct_destroy. To resolve this, move the IPS_CONFIRMED assignment after the table insertion but before the unlock. Pablo points out that the confirm-bit-store could be reordered to happen before hlist add resp. the timeout fixup, so switch to set_bit and before_atomic memory barrier to prevent this. It doesn't matter if other CPUs can observe a newly inserted entry right before the CONFIRMED bit was set: Such event cannot be distinguished from above "E is the old incarnation" case: the entry will be skipped. Also change nf_ct_should_gc() to first check the confirmed bit. The gc sequence is: 1. Check if entry has expired, if not skip to next entry 2. Obtain a reference to the expired entry. 3. Call nf_ct_should_gc() to double-check step 1. nf_ct_should_gc() is thus called only for entries that already failed an expiry check. After this patch, once the confirmed bit check pas ---truncated---
References
Impacted products
Vendor Product Version
Linux Linux Version: 1397af5bfd7d32b0cf2adb70a78c9a9e8f11d912
Version: 1397af5bfd7d32b0cf2adb70a78c9a9e8f11d912
Version: 1397af5bfd7d32b0cf2adb70a78c9a9e8f11d912
Version: 1397af5bfd7d32b0cf2adb70a78c9a9e8f11d912
Version: 1397af5bfd7d32b0cf2adb70a78c9a9e8f11d912
Version: 594cea2c09f7cd440d1ee1c4547d5bc6a646b0e4
Version: 5.18.13   ≤
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnetfilter: nf_conntrack: fix crash due to removal of uninitialised entry\n\nA crash in conntrack was reported while trying to unlink the conntrack\nentry from the hash bucket list:\n    [exception RIP: __nf_ct_delete_from_lists+172]\n    [..]\n #7 [ff539b5a2b043aa0] nf_ct_delete at ffffffffc124d421 [nf_conntrack]\n #8 [ff539b5a2b043ad0] nf_ct_gc_expired at ffffffffc124d999 [nf_conntrack]\n #9 [ff539b5a2b043ae0] __nf_conntrack_find_get at ffffffffc124efbc [nf_conntrack]\n    [..]\n\nThe nf_conn struct is marked as allocated from slab but appears to be in\na partially initialised state:\n\n ct hlist pointer is garbage; looks like the ct hash value\n (hence crash).\n ct-\u003estatus is equal to IPS_CONFIRMED|IPS_DYING, which is expected\n ct-\u003etimeout is 30000 (=30s), which is unexpected.\n\nEverything else looks like normal udp conntrack entry.  If we ignore\nct-\u003estatus and pretend its 0, the entry matches those that are newly\nallocated but not yet inserted into the hash:\n  - ct hlist pointers are overloaded and store/cache the raw tuple hash\n  - ct-\u003etimeout matches the relative time expected for a new udp flow\n    rather than the absolute \u0027jiffies\u0027 value.\n\nIf it were not for the presence of IPS_CONFIRMED,\n__nf_conntrack_find_get() would have skipped the entry.\n\nTheory is that we did hit following race:\n\ncpu x \t\t\tcpu y\t\t\tcpu z\n found entry E\t\tfound entry E\n E is expired\t\t\u003cpreemption\u003e\n nf_ct_delete()\n return E to rcu slab\n\t\t\t\t\tinit_conntrack\n\t\t\t\t\tE is re-inited,\n\t\t\t\t\tct-\u003estatus set to 0\n\t\t\t\t\treply tuplehash hnnode.pprev\n\t\t\t\t\tstores hash value.\n\ncpu y found E right before it was deleted on cpu x.\nE is now re-inited on cpu z.  cpu y was preempted before\nchecking for expiry and/or confirm bit.\n\n\t\t\t\t\t-\u003erefcnt set to 1\n\t\t\t\t\tE now owned by skb\n\t\t\t\t\t-\u003etimeout set to 30000\n\nIf cpu y were to resume now, it would observe E as\nexpired but would skip E due to missing CONFIRMED bit.\n\n\t\t\t\t\tnf_conntrack_confirm gets called\n\t\t\t\t\tsets: ct-\u003estatus |= CONFIRMED\n\t\t\t\t\tThis is wrong: E is not yet added\n\t\t\t\t\tto hashtable.\n\ncpu y resumes, it observes E as expired but CONFIRMED:\n\t\t\t\u003cresumes\u003e\n\t\t\tnf_ct_expired()\n\t\t\t -\u003e yes (ct-\u003etimeout is 30s)\n\t\t\tconfirmed bit set.\n\ncpu y will try to delete E from the hashtable:\n\t\t\tnf_ct_delete() -\u003e set DYING bit\n\t\t\t__nf_ct_delete_from_lists\n\nEven this scenario doesn\u0027t guarantee a crash:\ncpu z still holds the table bucket lock(s) so y blocks:\n\n\t\t\twait for spinlock held by z\n\n\t\t\t\t\tCONFIRMED is set but there is no\n\t\t\t\t\tguarantee ct will be added to hash:\n\t\t\t\t\t\"chaintoolong\" or \"clash resolution\"\n\t\t\t\t\tlogic both skip the insert step.\n\t\t\t\t\treply hnnode.pprev still stores the\n\t\t\t\t\thash value.\n\n\t\t\t\t\tunlocks spinlock\n\t\t\t\t\treturn NF_DROP\n\t\t\t\u003cunblocks, then\n\t\t\t crashes on hlist_nulls_del_rcu pprev\u003e\n\nIn case CPU z does insert the entry into the hashtable, cpu y will unlink\nE again right away but no crash occurs.\n\nWithout \u0027cpu y\u0027 race, \u0027garbage\u0027 hlist is of no consequence:\nct refcnt remains at 1, eventually skb will be free\u0027d and E gets\ndestroyed via: nf_conntrack_put -\u003e nf_conntrack_destroy -\u003e nf_ct_destroy.\n\nTo resolve this, move the IPS_CONFIRMED assignment after the table\ninsertion but before the unlock.\n\nPablo points out that the confirm-bit-store could be reordered to happen\nbefore hlist add resp. the timeout fixup, so switch to set_bit and\nbefore_atomic memory barrier to prevent this.\n\nIt doesn\u0027t matter if other CPUs can observe a newly inserted entry right\nbefore the CONFIRMED bit was set:\n\nSuch event cannot be distinguished from above \"E is the old incarnation\"\ncase: the entry will be skipped.\n\nAlso change nf_ct_should_gc() to first check the confirmed bit.\n\nThe gc sequence is:\n 1. Check if entry has expired, if not skip to next entry\n 2. Obtain a reference to the expired entry.\n 3. Call nf_ct_should_gc() to double-check step 1.\n\nnf_ct_should_gc() is thus called only for entries that already failed an\nexpiry check. After this patch, once the confirmed bit check pas\n---truncated---"
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 9.8,
            "baseSeverity": "CRITICAL",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - The entire code path (nf_conntrack_in \u2192 init_conntrack \u2192 __nf_conntrack_confirm \u2192 __nf_conntrack_find_get \u2192 nf_ct_gc_expired) executes in softirq context on received packets, with no syscall or local entry point required. Any Linux box doing stateful filtering or NAT \u2014 routers, firewalls, Docker/k8s hosts \u2014 reaches this purely from remote traffic.\nAC:L - The attacker drives every side of the race: a flood of short-lived UDP flows creates, expires, GCs and reallocates conntrack entries concurrently across CPUs, and duplicate same-5-tuple packets sent in parallel deliberately steer confirmation into the clash-resolution/chaintoolong paths that leave the uninitialised hlist pointer. The crash was observed spontaneously on production systems under ordinary load, confirming no attacker-uncontrollable precondition is needed.\nPR:N - No authentication or privilege of any kind is required \u2014 an unauthenticated remote host merely sends packets to or through the target. Conntrack entry allocation, confirmation and expiry are performed on behalf of arbitrary unsolicited traffic.\nUI:N - Exploitation requires nothing from any local user; packet processing happens automatically in softirq context on receipt.\nS:U - The corruption is confined to kernel memory within the same security authority; no VM, IOMMU or sandbox boundary is crossed.\nC:H - The refcount imbalance frees an nf_conn still referenced by a live skb, and SLAB_TYPESAFE_BY_RCU lets the attacker immediately reclaim it with an attacker-shaped nf_conn, giving a use-after-free read primitive over conntrack/NAT state and adjacent slab contents.\nI:H - `__hlist_nulls_del()` performs `WRITE_ONCE(*pprev, next)` where pprev is NULL or the raw attacker-influenced tuple hash \u2014 a wild kernel write \u2014 and the accompanying UAF permits heap grooming for control-flow hijack; the bug also silently unlinks valid live conntrack entries, corrupting firewall/NAT policy state.\nA:H - The reported real-world symptom is a kernel oops in __nf_ct_delete_from_lists from dereferencing the uninitialised hlist pprev, and the use-after-free independently causes panics; a remote attacker can trigger it repeatedly."
            }
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      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:02:11.183Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/a47ef874189d47f934d0809ae738886307c0ea22"
        },
        {
          "url": "https://git.kernel.org/stable/c/76179961c423cd698080b5e4d5583cf7f4fcdde9"
        },
        {
          "url": "https://git.kernel.org/stable/c/fc38c249c622ff5e3011b8845fd49dbfd9289afc"
        },
        {
          "url": "https://git.kernel.org/stable/c/938ce0e8422d3793fe30df2ed0e37f6bc0598379"
        },
        {
          "url": "https://git.kernel.org/stable/c/2d72afb340657f03f7261e9243b44457a9228ac7"
        }
      ],
      "title": "netfilter: nf_conntrack: fix crash due to removal of uninitialised entry",
      "x_generator": {
        "engine": "bippy-1.2.0"
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  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2025-38472",
    "datePublished": "2025-07-28T11:21:33.977Z",
    "dateReserved": "2025-04-16T04:51:24.021Z",
    "dateUpdated": "2026-08-05T12:02:11.183Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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