CVE-2026-46099 (GCVE-0-2026-46099)
Vulnerability from cvelistv5
Published
2026-05-27 12:59
Modified
2026-08-26 12:04
Severity ?
8.1 (High) - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels seg6_input_core() and rpl_input() call ip6_route_input() which sets a NOREF dst on the skb, then pass it to dst_cache_set_ip6() invoking dst_hold() unconditionally. On PREEMPT_RT, ksoftirqd is preemptible and a higher-priority task can release the underlying pcpu_rt between the lookup and the caching through a concurrent FIB lookup on a shared nexthop. Simplified race sequence: ksoftirqd/X higher-prio task (same CPU X) ----------- -------------------------------- seg6_input_core(,skb)/rpl_input(skb) dst_cache_get() -> miss ip6_route_input(skb) -> ip6_pol_route(,skb,flags) [RT6_LOOKUP_F_DST_NOREF in flags] -> FIB lookup resolves fib6_nh [nhid=N route] -> rt6_make_pcpu_route() [creates pcpu_rt, refcount=1] pcpu_rt->sernum = fib6_sernum [fib6_sernum=W] -> cmpxchg(fib6_nh.rt6i_pcpu, NULL, pcpu_rt) [slot was empty, store succeeds] -> skb_dst_set_noref(skb, dst) [dst is pcpu_rt, refcount still 1] rt_genid_bump_ipv6() -> bumps fib6_sernum [fib6_sernum from W to Z] ip6_route_output() -> ip6_pol_route() -> FIB lookup resolves fib6_nh [nhid=N] -> rt6_get_pcpu_route() pcpu_rt->sernum != fib6_sernum [W <> Z, stale] -> prev = xchg(rt6i_pcpu, NULL) -> dst_release(prev) [prev is pcpu_rt, refcount 1->0, dead] dst = skb_dst(skb) [dst is the dead pcpu_rt] dst_cache_set_ip6(dst) -> dst_hold() on dead dst -> WARN / use-after-free For the race to occur, ksoftirqd must be preemptible (PREEMPT_RT without PREEMPT_RT_NEEDS_BH_LOCK) and a concurrent task must be able to release the pcpu_rt. Shared nexthop objects provide such a path, as two routes pointing to the same nhid share the same fib6_nh and its rt6i_pcpu entry. Fix seg6_input_core() and rpl_input() by calling skb_dst_force() after ip6_route_input() to force the NOREF dst into a refcounted one before caching. The output path is not affected as ip6_route_output() already returns a refcounted dst.
References
Impacted products
Vendor Product Version
Linux Linux Version: af4a2209b1344939eaac11f269c261d347cbc3ee
Version: af4a2209b1344939eaac11f269c261d347cbc3ee
Version: af4a2209b1344939eaac11f269c261d347cbc3ee
Version: af4a2209b1344939eaac11f269c261d347cbc3ee
Version: af4a2209b1344939eaac11f269c261d347cbc3ee
Version: af4a2209b1344939eaac11f269c261d347cbc3ee
Version: af4a2209b1344939eaac11f269c261d347cbc3ee
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels\n\nseg6_input_core() and rpl_input() call ip6_route_input() which sets a\nNOREF dst on the skb, then pass it to dst_cache_set_ip6() invoking\ndst_hold() unconditionally.\nOn PREEMPT_RT, ksoftirqd is preemptible and a higher-priority task can\nrelease the underlying pcpu_rt between the lookup and the caching\nthrough a concurrent FIB lookup on a shared nexthop.\nSimplified race sequence:\n\n  ksoftirqd/X                       higher-prio task (same CPU X)\n  -----------                       --------------------------------\n  seg6_input_core(,skb)/rpl_input(skb)\n    dst_cache_get()\n      -\u003e miss\n    ip6_route_input(skb)\n      -\u003e ip6_pol_route(,skb,flags)\n         [RT6_LOOKUP_F_DST_NOREF in flags]\n        -\u003e FIB lookup resolves fib6_nh\n           [nhid=N route]\n        -\u003e rt6_make_pcpu_route()\n           [creates pcpu_rt, refcount=1]\n             pcpu_rt-\u003esernum = fib6_sernum\n             [fib6_sernum=W]\n           -\u003e cmpxchg(fib6_nh.rt6i_pcpu,\n                      NULL, pcpu_rt)\n              [slot was empty, store succeeds]\n      -\u003e skb_dst_set_noref(skb, dst)\n         [dst is pcpu_rt, refcount still 1]\n\n                                    rt_genid_bump_ipv6()\n                                      -\u003e bumps fib6_sernum\n                                         [fib6_sernum from W to Z]\n                                    ip6_route_output()\n                                      -\u003e ip6_pol_route()\n                                        -\u003e FIB lookup resolves fib6_nh\n                                           [nhid=N]\n                                        -\u003e rt6_get_pcpu_route()\n                                             pcpu_rt-\u003esernum != fib6_sernum\n                                             [W \u003c\u003e Z, stale]\n                                          -\u003e prev = xchg(rt6i_pcpu, NULL)\n                                          -\u003e dst_release(prev)\n                                             [prev is pcpu_rt,\n                                              refcount 1-\u003e0, dead]\n\n    dst = skb_dst(skb)\n    [dst is the dead pcpu_rt]\n    dst_cache_set_ip6(dst)\n      -\u003e dst_hold() on dead dst\n      -\u003e WARN / use-after-free\n\nFor the race to occur, ksoftirqd must be preemptible (PREEMPT_RT without\nPREEMPT_RT_NEEDS_BH_LOCK) and a concurrent task must be able to release\nthe pcpu_rt. Shared nexthop objects provide such a path, as two routes\npointing to the same nhid share the same fib6_nh and its rt6i_pcpu\nentry.\n\nFix seg6_input_core() and rpl_input() by calling skb_dst_force() after\nip6_route_input() to force the NOREF dst into a refcounted one before\ncaching.\nThe output path is not affected as ip6_route_output() already returns a\nrefcounted dst."
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              "value": "AV:N - The seg6/RPL lwtunnel input handlers run on the IPv6 packet receive/forward path (lwtunnel_input \u2192 seg6_input/rpl_input); a remote attacker sending IPv6 packets matching a configured SRv6/RPL-encap route reaches the vulnerable code.\nAC:H - The UAF manifests only on PREEMPT_RT kernels built without PREEMPT_RT_NEEDS_BH_LOCK (a rare, non-default configuration) and additionally requires winning a tight race against a higher-priority task\u0027s concurrent FIB lookup plus a sernum bump on a shared nexthop \u2014 conditions outside the attacker\u0027s control.\nPR:N - The triggering packets require no privileges; the SRv6/RPL route configuration is an environmental precondition (CAP_NET_ADMIN to set up), not a privilege the network attacker must hold.\nUI:N - The bug is triggered purely by inbound packet processing in softirq context; no victim interaction is needed.\nS:U - The use-after-free is confined to kernel route/dst objects within the kernel\u0027s own security authority; no boundary (VM/IOMMU) is crossed.\nC:H - The freed rt6_info/dst object is read back (rt6_get_cookie/dst dereferences) and can be reallocated with attacker-influenced heap content, enabling kernel memory disclosure.\nI:H - A use-after-free on a route object allows heap-spray reallocation and write/control primitives via the dangling dst that is subsequently used for routing decisions.\nA:H - dst_hold() on the dead object triggers a WARN and the use-after-free of a freed route can cause an oops/panic, crashing the kernel."
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Author Source Type Date

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
  • Confirmed: The vulnerability is confirmed from an analyst perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
  • Patched: This vulnerability was successfully patched by the user reporting the sighting.
  • Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
  • Not confirmed: The user expresses doubt about the veracity of the vulnerability.
  • Not patched: This vulnerability was not successfully patched by the user reporting the sighting.