CVE-2026-63981
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:
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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.
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.
Detalles técnicos trazas, registros y código del informe original
tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat) 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
CVSS
NVD no ha asignado puntuación CVSS a esta CVE (habitual desde el cambio de política de abril de 2026).
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.23%
- Percentil entre todas las CVEs puntuadas: 12
- Fecha de la puntuación: 6/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
Tecnologías afectadas (1)
⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.
Referencias
JSON original (NVD)
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{
"id": "CVE-2026-63981",
"cveTags": [],
"metrics": {},
"affected": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"affectedData": [
{
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"product": "Linux",
"versions": [
{
"status": "affected",
"version": "906736728cea480a85803c67fafb1b0e78491922",
"lessThan": "25fc9352590f5ef21ebf290432bd768b336693bc",
"versionType": "git"
},
{
"status": "affected",
"version": "fe946a751d9b52b7c45ca34899723b314b79b249",
"lessThan": "34457de389fb64a01fdcc71177dfebe65fd2d362",
"versionType": "git"
},
{
"status": "affected",
"version": "fe946a751d9b52b7c45ca34899723b314b79b249",
"lessThan": "a005fa5d7502eefec7ee6e1c01adadc06de2f9ad",
"versionType": "git"
}
],
"programFiles": [
"net/sched/act_mirred.c"
],
"defaultStatus": "unaffected"
},
{
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"product": "Linux",
"versions": [
{
"status": "affected",
"version": "6.19"
},
{
"status": "unaffected",
"version": "0",
"lessThan": "6.19",
"versionType": "semver"
},
{
"status": "unaffected",
"version": "7.0.12",
"versionType": "semver",
"lessThanOrEqual": "7.0.*"
},
{
"status": "unaffected",
"version": "7.1",
"versionType": "original_commit_for_fix",
"lessThanOrEqual": "*"
}
],
"programFiles": [
"net/sched/act_mirred.c"
],
"defaultStatus": "affected"
}
]
}
],
"published": "2026-07-19T16:17:17.283",
"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"
}
],
"vulnStatus": "Awaiting Analysis",
"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's egress TC re-enters tcf_mirred_act() via blockcast and\nmirrors back to A, creating an unbounded recursion loop:\n\n tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit\n -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (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."
}
],
"lastModified": "2026-07-30T14:51:11.223",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}