{"schema_version":"1.9.0","id":"CVE-2026-63981","published":"2026-07-19T14:56:04.580Z","modified":"2026-08-12T03:51:33.714391782Z","summary":"net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow","details":"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.","affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"906736728cea480a85803c67fafb1b0e78491922"},{"fixed":"25fc9352590f5ef21ebf290432bd768b336693bc"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"fe946a751d9b52b7c45ca34899723b314b79b249"},{"fixed":"34457de389fb64a01fdcc71177dfebe65fd2d362"},{"fixed":"a005fa5d7502eefec7ee6e1c01adadc06de2f9ad"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-63981.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.0.12"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-63981.json"}}],"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/25fc9352590f5ef21ebf290432bd768b336693bc"},{"type":"WEB","url":"https://git.kernel.org/stable/c/34457de389fb64a01fdcc71177dfebe65fd2d362"},{"type":"WEB","url":"https://git.kernel.org/stable/c/a005fa5d7502eefec7ee6e1c01adadc06de2f9ad"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63981.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63981"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63981.json"}}