CVE-2026-63981 (GCVE-0-2026-63981)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:56 – Updated: 2026-07-19 14:56
VLAI
Title
net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow
Summary
In the Linux kernel, the following vulnerability has been resolved: net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow tcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4) to prevent deep recursion. However, when the action uses blockcast (tcfm_blockid != 0), the function returns at the tcf_blockcast() call BEFORE reaching the counter increment. As a result, the recursion counter never advances and the limit check is entirely bypassed. When two devices share a TC egress block with a mirred blockcast rule, a packet egressing on device A is mirrored to device B via blockcast; device B's egress TC re-enters tcf_mirred_act() via blockcast and mirrors back to A, creating an unbounded recursion loop: tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat) This recursion continues until the kernel stack overflows. The bug is reachable from an unprivileged user via unshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant CAP_NET_ADMIN in the new network namespace, which is sufficient to create dummy devices, attach clsact qdiscs with shared blocks, and install mirred blockcast filters. BUG: TASK stack guard page was hit at ffffc90000b7fff8 Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410 RIP: 0010:xas_find+0x17/0x480 Call Trace: xa_find+0x17b/0x1d0 tcf_mirred_act+0x640/0x1060 tcf_action_exec+0x400/0x530 basic_classify+0x128/0x1d0 tcf_classify+0xd83/0x1150 tc_run+0x328/0x620 __dev_queue_xmit+0x797/0x3100 tcf_mirred_to_dev+0x7b1/0xf70 tcf_mirred_act+0x68a/0x1060 [repeating ~30+ times until stack overflow] Kernel panic - not syncing: Fatal exception in interrupt Fix this by incrementing sched_mirred_nest before calling tcf_blockcast() and decrementing it on return, mirroring the non-blockcast path. This ensures subsequent recursive entries see the updated counter and are correctly limited by MIRRED_NEST_LIMIT.
Severity
No CVSS data available.
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 906736728cea480a85803c67fafb1b0e78491922 , < 25fc9352590f5ef21ebf290432bd768b336693bc (git)
Affected: fe946a751d9b52b7c45ca34899723b314b79b249 , < 34457de389fb64a01fdcc71177dfebe65fd2d362 (git)
Affected: fe946a751d9b52b7c45ca34899723b314b79b249 , < a005fa5d7502eefec7ee6e1c01adadc06de2f9ad (git)
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Linux Linux Affected: 6.19
Unaffected: 0 , < 6.19 (semver)
Unaffected: 7.0.12 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
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{
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      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "net/sched/act_mirred.c"
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              "versionType": "semver"
            },
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              "status": "unaffected",
              "version": "7.1",
              "versionType": "original_commit_for_fix"
            }
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              "operator": "OR"
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow\n\ntcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4)\nto prevent deep recursion.  However, when the action uses blockcast\n(tcfm_blockid != 0), the function returns at the tcf_blockcast() call\nBEFORE reaching the counter increment.  As a result, the recursion\ncounter never advances and the limit check is entirely bypassed.\n\nWhen two devices share a TC egress block with a mirred blockcast rule,\na packet egressing on device A is mirrored to device B via blockcast;\ndevice B\u0027s egress TC re-enters tcf_mirred_act() via blockcast and\nmirrors back to A, creating an unbounded recursion loop:\n\n  tcf_mirred_act -\u003e tcf_blockcast -\u003e tcf_mirred_to_dev -\u003e dev_queue_xmit\n  -\u003e sch_handle_egress -\u003e tcf_classify -\u003e tcf_mirred_act -\u003e (repeat)\n\nThis recursion continues until the kernel stack overflows.\n\nThe bug is reachable from an unprivileged user via\nunshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant\nCAP_NET_ADMIN in the new network namespace, which is sufficient to\ncreate dummy devices, attach clsact qdiscs with shared blocks, and\ninstall mirred blockcast filters.\n\n BUG: TASK stack guard page was hit at ffffc90000b7fff8\n Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI\n CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410\n RIP: 0010:xas_find+0x17/0x480\n Call Trace:\n  xa_find+0x17b/0x1d0\n  tcf_mirred_act+0x640/0x1060\n  tcf_action_exec+0x400/0x530\n  basic_classify+0x128/0x1d0\n  tcf_classify+0xd83/0x1150\n  tc_run+0x328/0x620\n  __dev_queue_xmit+0x797/0x3100\n  tcf_mirred_to_dev+0x7b1/0xf70\n  tcf_mirred_act+0x68a/0x1060\n  [repeating ~30+ times until stack overflow]\n Kernel panic - not syncing: Fatal exception in interrupt\n\nFix this by incrementing sched_mirred_nest before calling\ntcf_blockcast() and decrementing it on return, mirroring the\nnon-blockcast path.  This ensures subsequent recursive entries see the\nupdated counter and are correctly limited by MIRRED_NEST_LIMIT."
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-07-19T14:56:04.580Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
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          "url": "https://git.kernel.org/stable/c/25fc9352590f5ef21ebf290432bd768b336693bc"
        },
        {
          "url": "https://git.kernel.org/stable/c/34457de389fb64a01fdcc71177dfebe65fd2d362"
        },
        {
          "url": "https://git.kernel.org/stable/c/a005fa5d7502eefec7ee6e1c01adadc06de2f9ad"
        }
      ],
      "title": "net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow",
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  "cveMetadata": {
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63981",
    "datePublished": "2026-07-19T14:56:04.580Z",
    "dateReserved": "2026-07-19T07:54:57.025Z",
    "dateUpdated": "2026-07-19T14:56:04.580Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-63981",
      "date": "2026-07-21",
      "epss": "0.00187",
      "percentile": "0.08583"
    },
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-63981\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2026-07-19T16:17:17.283\",\"lastModified\":\"2026-07-19T16:17:17.283\",\"vulnStatus\":\"Received\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"In the Linux kernel, the following vulnerability has been resolved:\\n\\nnet/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow\\n\\ntcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4)\\nto prevent deep recursion.  However, when the action uses blockcast\\n(tcfm_blockid != 0), the function returns at the tcf_blockcast() call\\nBEFORE reaching the counter increment.  As a result, the recursion\\ncounter never advances and the limit check is entirely bypassed.\\n\\nWhen two devices share a TC egress block with a mirred blockcast rule,\\na packet egressing on device A is mirrored to device B via blockcast;\\ndevice B\u0027s egress TC re-enters tcf_mirred_act() via blockcast and\\nmirrors back to A, creating an unbounded recursion loop:\\n\\n  tcf_mirred_act -\u003e tcf_blockcast -\u003e tcf_mirred_to_dev -\u003e dev_queue_xmit\\n  -\u003e sch_handle_egress -\u003e tcf_classify -\u003e tcf_mirred_act -\u003e (repeat)\\n\\nThis recursion continues until the kernel stack overflows.\\n\\nThe bug is reachable from an unprivileged user via\\nunshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant\\nCAP_NET_ADMIN in the new network namespace, which is sufficient to\\ncreate dummy devices, attach clsact qdiscs with shared blocks, and\\ninstall mirred blockcast filters.\\n\\n BUG: TASK stack guard page was hit at ffffc90000b7fff8\\n Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI\\n CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410\\n RIP: 0010:xas_find+0x17/0x480\\n Call Trace:\\n  xa_find+0x17b/0x1d0\\n  tcf_mirred_act+0x640/0x1060\\n  tcf_action_exec+0x400/0x530\\n  basic_classify+0x128/0x1d0\\n  tcf_classify+0xd83/0x1150\\n  tc_run+0x328/0x620\\n  __dev_queue_xmit+0x797/0x3100\\n  tcf_mirred_to_dev+0x7b1/0xf70\\n  tcf_mirred_act+0x68a/0x1060\\n  [repeating ~30+ times until stack overflow]\\n Kernel panic - not syncing: Fatal exception in interrupt\\n\\nFix this by incrementing sched_mirred_nest before calling\\ntcf_blockcast() and decrementing it on return, mirroring the\\nnon-blockcast path.  This ensures subsequent recursive entries see the\\nupdated counter and are correctly limited by MIRRED_NEST_LIMIT.\"}],\"affected\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"affectedData\":[{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"unaffected\",\"programFiles\":[\"net/sched/act_mirred.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"906736728cea480a85803c67fafb1b0e78491922\",\"lessThan\":\"25fc9352590f5ef21ebf290432bd768b336693bc\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"fe946a751d9b52b7c45ca34899723b314b79b249\",\"lessThan\":\"34457de389fb64a01fdcc71177dfebe65fd2d362\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"fe946a751d9b52b7c45ca34899723b314b79b249\",\"lessThan\":\"a005fa5d7502eefec7ee6e1c01adadc06de2f9ad\",\"versionType\":\"git\",\"status\":\"affected\"}]},{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"affected\",\"programFiles\":[\"net/sched/act_mirred.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"6.19\",\"status\":\"affected\"},{\"version\":\"0\",\"lessThan\":\"6.19\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.0.12\",\"lessThanOrEqual\":\"7.0.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.1\",\"lessThanOrEqual\":\"*\",\"versionType\":\"original_commit_for_fix\",\"status\":\"unaffected\"}]}]}],\"metrics\":{},\"references\":[{\"url\":\"https://git.kernel.org/stable/c/25fc9352590f5ef21ebf290432bd768b336693bc\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/34457de389fb64a01fdcc71177dfebe65fd2d362\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/a005fa5d7502eefec7ee6e1c01adadc06de2f9ad\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"}]}}"
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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