CVE-2021-46936 (GCVE-0-2021-46936)

Vulnerability from cvelistv5 – Published: 2024-02-27 09:44 – Updated: 2026-08-05 08:45
VLAI
Title
net: fix use-after-free in tw_timer_handler
Summary
In the Linux kernel, the following vulnerability has been resolved: net: fix use-after-free in tw_timer_handler A real world panic issue was found as follow in Linux 5.4. BUG: unable to handle page fault for address: ffffde49a863de28 PGD 7e6fe62067 P4D 7e6fe62067 PUD 7e6fe63067 PMD f51e064067 PTE 0 RIP: 0010:tw_timer_handler+0x20/0x40 Call Trace: <IRQ> call_timer_fn+0x2b/0x120 run_timer_softirq+0x1ef/0x450 __do_softirq+0x10d/0x2b8 irq_exit+0xc7/0xd0 smp_apic_timer_interrupt+0x68/0x120 apic_timer_interrupt+0xf/0x20 This issue was also reported since 2017 in the thread [1], unfortunately, the issue was still can be reproduced after fixing DCCP. The ipv4_mib_exit_net is called before tcp_sk_exit_batch when a net namespace is destroyed since tcp_sk_ops is registered befrore ipv4_mib_ops, which means tcp_sk_ops is in the front of ipv4_mib_ops in the list of pernet_list. There will be a use-after-free on net->mib.net_statistics in tw_timer_handler after ipv4_mib_exit_net if there are some inflight time-wait timers. This bug is not introduced by commit f2bf415cfed7 ("mib: add net to NET_ADD_STATS_BH") since the net_statistics is a global variable instead of dynamic allocation and freeing. Actually, commit 61a7e26028b9 ("mib: put net statistics on struct net") introduces the bug since it put net statistics on struct net and free it when net namespace is destroyed. Moving init_ipv4_mibs() to the front of tcp_init() to fix this bug and replace pr_crit() with panic() since continuing is meaningless when init_ipv4_mibs() fails. [1] https://groups.google.com/g/syzkaller/c/p1tn-_Kc6l4/m/smuL_FMAAgAJ?pli=1
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
SSVC
Exploitation: none Automatable: no Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2024-09-10 16:01 UTC
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 6665 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < 15579e1301f856ad9385d720c9267c11032a5022 (git)
Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < e73164e89d1be561228a4534e1091369ee4ba41a (git)
Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < 5c2fe20ad37ff56070ae0acb34152333976929b4 (git)
Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < a8e1944b44f94f5c5f530e434c5eaee787254566 (git)
Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < fe5838c22b986c1190f1dce9aa09bf6a491c1a69 (git)
Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < 2386e81a1d277f540e1285565c9d41d531bb69d4 (git)
Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < 08eacbd141e2495d2fcdde84358a06c4f95cbb13 (git)
Affected: 61a7e26028b94805fd686a6dc9dbd9941f8f19b0 , < e22e45fc9e41bf9fcc1e92cfb78eb92786728ef0 (git)
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Linux Linux Affected: 2.6.27
Unaffected: 0 , < 2.6.27 (semver)
Unaffected: 4.4.298 , ≤ 4.4.* (semver)
Unaffected: 4.9.296 , ≤ 4.9.* (semver)
Unaffected: 4.14.261 , ≤ 4.14.* (semver)
Unaffected: 4.19.224 , ≤ 4.19.* (semver)
Unaffected: 5.4.170 , ≤ 5.4.* (semver)
Unaffected: 5.10.90 , ≤ 5.10.* (semver)
Unaffected: 5.15.13 , ≤ 5.15.* (semver)
Unaffected: 5.16 , ≤ * (original_commit_for_fix)
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            "value": "En el kernel de Linux, se resolvi\u00f3 la siguiente vulnerabilidad: net: corrige use-after-free en tw_timer_handler Se encontr\u00f3 un problema de p\u00e1nico en el mundo real como se muestra a continuaci\u00f3n en Linux 5.4. ERROR: no se puede manejar el error de p\u00e1gina para la direcci\u00f3n: ffffde49a863de28 PGD 7e6fe62067 P4D 7e6fe62067 PUD 7e6fe63067 PMD f51e064067 PTE 0 RIP: 0010:tw_timer_handler+0x20/0x40 Seguimiento de llamadas:  call_timer_fn+0x2b/ 0x120 run_timer_softirq+0x1ef/0x450 __do_softirq+0x10d/ 0x2b8 irq_exit+0xc7/0xd0 smp_apic_timer_interrupt+0x68/0x120 apic_timer_interrupt+0xf/0x20 Este problema tambi\u00e9n se inform\u00f3 desde 2017 en el hilo [1], desafortunadamente, el problema a\u00fan se puede reproducir despu\u00e9s de corregir DCCP. ipv4_mib_exit_net se llama antes de tcp_sk_exit_batch cuando se destruye un espacio de nombres de red, ya que tcp_sk_ops est\u00e1 registrado antes de ipv4_mib_ops, lo que significa que tcp_sk_ops est\u00e1 al frente de ipv4_mib_ops en la lista de pernet_list. Habr\u00e1 un use-after-free en net-\u0026gt;mib.net_statistics en tw_timer_handler despu\u00e9s de ipv4_mib_exit_net si hay algunos temporizadores de espera a bordo. Este error no se introduce mediante la confirmaci\u00f3n f2bf415cfed7 (\"mib: add net to NET_ADD_STATS_BH\") ya que net_statistics es una variable global en lugar de una asignaci\u00f3n y liberaci\u00f3n din\u00e1micas. En realidad, la confirmaci\u00f3n 61a7e26028b9 (\"mib: poner estad\u00edsticas de red en struct net\") introduce el error ya que coloca estad\u00edsticas de red en struct net y las libera cuando se destruye el espacio de nombres de red. Mover init_ipv4_mibs() al frente de tcp_init() para corregir este error y reemplazar pr_crit() con p\u00e1nico() ya que continuar no tiene sentido cuando init_ipv4_mibs() falla. [1] https://groups.google.com/g/syzkaller/c/p1tn-_Kc6l4/m/smuL_FMAAgAJ?pli=1"
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                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "4.19.224",
                      "versionStartIncluding": "2.6.27",
                      "vulnerable": true
                    },
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.4.170",
                      "versionStartIncluding": "2.6.27",
                      "vulnerable": true
                    },
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.10.90",
                      "versionStartIncluding": "2.6.27",
                      "vulnerable": true
                    },
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.15.13",
                      "versionStartIncluding": "2.6.27",
                      "vulnerable": true
                    },
                    {
                      "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                      "versionEndExcluding": "5.16",
                      "versionStartIncluding": "2.6.27",
                      "vulnerable": true
                    }
                  ],
                  "negate": false,
                  "operator": "OR"
                }
              ]
            }
          ],
          "descriptions": [
            {
              "lang": "en",
              "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet: fix use-after-free in tw_timer_handler\n\nA real world panic issue was found as follow in Linux 5.4.\n\n    BUG: unable to handle page fault for address: ffffde49a863de28\n    PGD 7e6fe62067 P4D 7e6fe62067 PUD 7e6fe63067 PMD f51e064067 PTE 0\n    RIP: 0010:tw_timer_handler+0x20/0x40\n    Call Trace:\n     \u003cIRQ\u003e\n     call_timer_fn+0x2b/0x120\n     run_timer_softirq+0x1ef/0x450\n     __do_softirq+0x10d/0x2b8\n     irq_exit+0xc7/0xd0\n     smp_apic_timer_interrupt+0x68/0x120\n     apic_timer_interrupt+0xf/0x20\n\nThis issue was also reported since 2017 in the thread [1],\nunfortunately, the issue was still can be reproduced after fixing\nDCCP.\n\nThe ipv4_mib_exit_net is called before tcp_sk_exit_batch when a net\nnamespace is destroyed since tcp_sk_ops is registered befrore\nipv4_mib_ops, which means tcp_sk_ops is in the front of ipv4_mib_ops\nin the list of pernet_list. There will be a use-after-free on\nnet-\u003emib.net_statistics in tw_timer_handler after ipv4_mib_exit_net\nif there are some inflight time-wait timers.\n\nThis bug is not introduced by commit f2bf415cfed7 (\"mib: add net to\nNET_ADD_STATS_BH\") since the net_statistics is a global variable\ninstead of dynamic allocation and freeing. Actually, commit\n61a7e26028b9 (\"mib: put net statistics on struct net\") introduces\nthe bug since it put net statistics on struct net and free it when\nnet namespace is destroyed.\n\nMoving init_ipv4_mibs() to the front of tcp_init() to fix this bug\nand replace pr_crit() with panic() since continuing is meaningless\nwhen init_ipv4_mibs() fails.\n\n[1] https://groups.google.com/g/syzkaller/c/p1tn-_Kc6l4/m/smuL_FMAAgAJ?pli=1"
            }
          ],
          "metrics": [
            {
              "cvssV3_1": {
                "baseScore": 7.8,
                "baseSeverity": "HIGH",
                "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
                "version": "3.1"
              },
              "scenarios": [
                {
                  "lang": "en",
                  "value": "AV:L - The bug is triggered by destroying a network namespace while TCP TIME_WAIT timers remain armed, which requires local syscalls (clone/unshare, socket connect/close); a remote packet alone cannot tear down the netns or hit this teardown ordering.\nAC:L - An attacker fully controls both sides\u2014creating TIME_WAIT sockets (including short-timeout tw_kill paths) and destroying the netns\u2014so the race between ipv4_mib_exit_net freeing stats and tcp_sk_exit_batch purging timers is attacker-driven and has been reproduced in the wild and by syzkaller.\nPR:L - An unprivileged process can create a user+net namespace (CLONE_NEWUSER|CLONE_NEWNET), open loopback TCP connections that enter TIME_WAIT, and exit the netns; CAP_SYS_ADMIN inside the user namespace is sufficient and does not require host root.\nUI:N - Exploitation requires no victim action; the attacker creates the sockets and destroys the namespace themselves.\nS:U - Impact is confined to the host kernel\u2019s networking subsystem authority; this is a standard kernel UAF/privilege-escalation class bug, not a VM/IOMMU/sandbox boundary escape.\nC:H - This is a use-after-free on net-\u003emib.net_statistics in tw_timer_handler; UAF of freed kernel heap/percpu memory can be leveraged for information disclosure once the object is reused.\nI:H - tw_timer_handler performs __NET_INC_STATS writes into the already-freed percpu MIB, yielding a memory-corruption write primitive that can be escalated via heap reuse to integrity compromise or code execution.\nA:H - The vulnerability causes a real-world kernel page-fault panic in softirq context (tw_timer_handler), so availability impact is a full kernel oops/panic."
                }
              ]
            }
          ],
          "providerMetadata": {
            "dateUpdated": "2026-08-05T08:45:10.624Z",
            "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "shortName": "Linux"
          },
          "references": [
            {
              "url": "https://git.kernel.org/stable/c/15579e1301f856ad9385d720c9267c11032a5022"
            },
            {
              "url": "https://git.kernel.org/stable/c/e73164e89d1be561228a4534e1091369ee4ba41a"
            },
            {
              "url": "https://git.kernel.org/stable/c/5c2fe20ad37ff56070ae0acb34152333976929b4"
            },
            {
              "url": "https://git.kernel.org/stable/c/a8e1944b44f94f5c5f530e434c5eaee787254566"
            },
            {
              "url": "https://git.kernel.org/stable/c/fe5838c22b986c1190f1dce9aa09bf6a491c1a69"
            },
            {
              "url": "https://git.kernel.org/stable/c/2386e81a1d277f540e1285565c9d41d531bb69d4"
            },
            {
              "url": "https://git.kernel.org/stable/c/08eacbd141e2495d2fcdde84358a06c4f95cbb13"
            },
            {
              "url": "https://git.kernel.org/stable/c/e22e45fc9e41bf9fcc1e92cfb78eb92786728ef0"
            }
          ],
          "title": "net: fix use-after-free in tw_timer_handler",
          "x_generator": {
            "engine": "bippy-1.2.0"
          }
        }
      },
      "cveMetadata": {
        "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "assignerShortName": "Linux",
        "cveId": "CVE-2021-46936",
        "datePublished": "2024-02-27T09:44:02.758Z",
        "dateReserved": "2024-02-25T13:45:52.720Z",
        "dateUpdated": "2026-08-05T08:45:10.624Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}



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