CVE-2025-38468 (GCVE-0-2025-38468)

Vulnerability from cvelistv5 – Published: 2025-07-28 11:12 – Updated: 2025-11-03 17:38
VLAI?
Title
net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree
Summary
In the Linux kernel, the following vulnerability has been resolved: net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree htb_lookup_leaf has a BUG_ON that can trigger with the following: tc qdisc del dev lo root tc qdisc add dev lo root handle 1: htb default 1 tc class add dev lo parent 1: classid 1:1 htb rate 64bit tc qdisc add dev lo parent 1:1 handle 2: netem tc qdisc add dev lo parent 2:1 handle 3: blackhole ping -I lo -c1 -W0.001 127.0.0.1 The root cause is the following: 1. htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on the selected leaf qdisc 2. netem_dequeue calls enqueue on the child qdisc 3. blackhole_enqueue drops the packet and returns a value that is not just NET_XMIT_SUCCESS 4. Because of this, netem_dequeue calls qdisc_tree_reduce_backlog, and since qlen is now 0, it calls htb_qlen_notify -> htb_deactivate -> htb_deactiviate_prios -> htb_remove_class_from_row -> htb_safe_rb_erase 5. As this is the only class in the selected hprio rbtree, __rb_change_child in __rb_erase_augmented sets the rb_root pointer to NULL 6. Because blackhole_dequeue returns NULL, netem_dequeue returns NULL, which causes htb_dequeue_tree to call htb_lookup_leaf with the same hprio rbtree, and fail the BUG_ON The function graph for this scenario is shown here: 0) | htb_enqueue() { 0) + 13.635 us | netem_enqueue(); 0) 4.719 us | htb_activate_prios(); 0) # 2249.199 us | } 0) | htb_dequeue() { 0) 2.355 us | htb_lookup_leaf(); 0) | netem_dequeue() { 0) + 11.061 us | blackhole_enqueue(); 0) | qdisc_tree_reduce_backlog() { 0) | qdisc_lookup_rcu() { 0) 1.873 us | qdisc_match_from_root(); 0) 6.292 us | } 0) 1.894 us | htb_search(); 0) | htb_qlen_notify() { 0) 2.655 us | htb_deactivate_prios(); 0) 6.933 us | } 0) + 25.227 us | } 0) 1.983 us | blackhole_dequeue(); 0) + 86.553 us | } 0) # 2932.761 us | qdisc_warn_nonwc(); 0) | htb_lookup_leaf() { 0) | BUG_ON(); ------------------------------------------ The full original bug report can be seen here [1]. We can fix this just by returning NULL instead of the BUG_ON, as htb_dequeue_tree returns NULL when htb_lookup_leaf returns NULL. [1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4wmGvNk2ceHjKb8ORnzw0cGwfmVu42g9dL7XyJLf1NEzaztboTWcm0Ogxuojoeo=@willsroot.io/
Severity ?
No CVSS data available.
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < fed3570e548a6c9f95c5f4c9e1a7afc1679fd90d (git)
Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < 5c0506cd1b1a3b145bda2612bbf7fe78d186c355 (git)
Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < 850226aef8d28a00cf966ef26d2f8f2bff344535 (git)
Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < 890a5d423ef0a7bd13447ceaffad21189f557301 (git)
Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < 7ff2d83ecf2619060f30ecf9fad4f2a700fca344 (git)
Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < e5c480dc62a3025b8428d4818e722da30ad6804f (git)
Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < 3691f84269a23f7edd263e9b6edbc27b7ae332f4 (git)
Affected: 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 , < 0e1d5d9b5c5966e2e42e298670808590db5ed628 (git)
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    Linux Linux Affected: 2.6.29
Unaffected: 0 , < 2.6.29 (semver)
Unaffected: 5.4.297 , ≤ 5.4.* (semver)
Unaffected: 5.10.241 , ≤ 5.10.* (semver)
Unaffected: 5.15.190 , ≤ 5.15.* (semver)
Unaffected: 6.1.147 , ≤ 6.1.* (semver)
Unaffected: 6.6.100 , ≤ 6.6.* (semver)
Unaffected: 6.12.40 , ≤ 6.12.* (semver)
Unaffected: 6.15.8 , ≤ 6.15.* (semver)
Unaffected: 6.16 , ≤ * (original_commit_for_fix)
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree\n\nhtb_lookup_leaf has a BUG_ON that can trigger with the following:\n\ntc qdisc del dev lo root\ntc qdisc add dev lo root handle 1: htb default 1\ntc class add dev lo parent 1: classid 1:1 htb rate 64bit\ntc qdisc add dev lo parent 1:1 handle 2: netem\ntc qdisc add dev lo parent 2:1 handle 3: blackhole\nping -I lo -c1 -W0.001 127.0.0.1\n\nThe root cause is the following:\n\n1. htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on\n   the selected leaf qdisc\n2. netem_dequeue calls enqueue on the child qdisc\n3. blackhole_enqueue drops the packet and returns a value that is not\n   just NET_XMIT_SUCCESS\n4. Because of this, netem_dequeue calls qdisc_tree_reduce_backlog, and\n   since qlen is now 0, it calls htb_qlen_notify -\u003e htb_deactivate -\u003e\n   htb_deactiviate_prios -\u003e htb_remove_class_from_row -\u003e htb_safe_rb_erase\n5. As this is the only class in the selected hprio rbtree,\n   __rb_change_child in __rb_erase_augmented sets the rb_root pointer to\n   NULL\n6. Because blackhole_dequeue returns NULL, netem_dequeue returns NULL,\n   which causes htb_dequeue_tree to call htb_lookup_leaf with the same\n   hprio rbtree, and fail the BUG_ON\n\nThe function graph for this scenario is shown here:\n 0)               |  htb_enqueue() {\n 0) + 13.635 us   |    netem_enqueue();\n 0)   4.719 us    |    htb_activate_prios();\n 0) # 2249.199 us |  }\n 0)               |  htb_dequeue() {\n 0)   2.355 us    |    htb_lookup_leaf();\n 0)               |    netem_dequeue() {\n 0) + 11.061 us   |      blackhole_enqueue();\n 0)               |      qdisc_tree_reduce_backlog() {\n 0)               |        qdisc_lookup_rcu() {\n 0)   1.873 us    |          qdisc_match_from_root();\n 0)   6.292 us    |        }\n 0)   1.894 us    |        htb_search();\n 0)               |        htb_qlen_notify() {\n 0)   2.655 us    |          htb_deactivate_prios();\n 0)   6.933 us    |        }\n 0) + 25.227 us   |      }\n 0)   1.983 us    |      blackhole_dequeue();\n 0) + 86.553 us   |    }\n 0) # 2932.761 us |    qdisc_warn_nonwc();\n 0)               |    htb_lookup_leaf() {\n 0)               |      BUG_ON();\n ------------------------------------------\n\nThe full original bug report can be seen here [1].\n\nWe can fix this just by returning NULL instead of the BUG_ON,\nas htb_dequeue_tree returns NULL when htb_lookup_leaf returns\nNULL.\n\n[1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4wmGvNk2ceHjKb8ORnzw0cGwfmVu42g9dL7XyJLf1NEzaztboTWcm0Ogxuojoeo=@willsroot.io/"
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        "dateUpdated": "2025-08-28T14:43:07.848Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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          "url": "https://git.kernel.org/stable/c/0e1d5d9b5c5966e2e42e298670808590db5ed628"
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      "title": "net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree",
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    "cveId": "CVE-2025-38468",
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    "dateReserved": "2025-04-16T04:51:24.020Z",
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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
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  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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