{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2025-39677","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","state":"PUBLISHED","assignerShortName":"Linux","dateReserved":"2025-04-16T07:20:57.112Z","datePublished":"2025-09-05T17:20:43.145Z","dateUpdated":"2026-05-11T21:34:07.205Z"},"containers":{"cna":{"providerMetadata":{"orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","shortName":"Linux","dateUpdated":"2026-05-11T21:34:07.205Z"},"descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: Fix backlog accounting in qdisc_dequeue_internal\n\nThis issue applies for the following qdiscs: hhf, fq, fq_codel, and\nfq_pie, and occurs in their change handlers when adjusting to the new\nlimit. The problem is the following in the values passed to the\nsubsequent qdisc_tree_reduce_backlog call given a tbf parent:\n\n   When the tbf parent runs out of tokens, skbs of these qdiscs will\n   be placed in gso_skb. Their peek handlers are qdisc_peek_dequeued,\n   which accounts for both qlen and backlog. However, in the case of\n   qdisc_dequeue_internal, ONLY qlen is accounted for when pulling\n   from gso_skb. This means that these qdiscs are missing a\n   qdisc_qstats_backlog_dec when dropping packets to satisfy the\n   new limit in their change handlers.\n\n   One can observe this issue with the following (with tc patched to\n   support a limit of 0):\n\n   export TARGET=fq\n   tc qdisc del dev lo root\n   tc qdisc add dev lo root handle 1: tbf rate 8bit burst 100b latency 1ms\n   tc qdisc replace dev lo handle 3: parent 1:1 $TARGET limit 1000\n   echo ''; echo 'add child'; tc -s -d qdisc show dev lo\n   ping -I lo -f -c2 -s32 -W0.001 127.0.0.1 2>&1 >/dev/null\n   echo ''; echo 'after ping'; tc -s -d qdisc show dev lo\n   tc qdisc change dev lo handle 3: parent 1:1 $TARGET limit 0\n   echo ''; echo 'after limit drop'; tc -s -d qdisc show dev lo\n   tc qdisc replace dev lo handle 2: parent 1:1 sfq\n   echo ''; echo 'post graft'; tc -s -d qdisc show dev lo\n\n   The second to last show command shows 0 packets but a positive\n   number (74) of backlog bytes. The problem becomes clearer in the\n   last show command, where qdisc_purge_queue triggers\n   qdisc_tree_reduce_backlog with the positive backlog and causes an\n   underflow in the tbf parent's backlog (4096 Mb instead of 0).\n\nTo fix this issue, the codepath for all clients of qdisc_dequeue_internal\nhas been simplified: codel, pie, hhf, fq, fq_pie, and fq_codel.\nqdisc_dequeue_internal handles the backlog adjustments for all cases that\ndo not directly use the dequeue handler.\n\nThe old fq_codel_change limit adjustment loop accumulated the arguments to\nthe subsequent qdisc_tree_reduce_backlog call through the cstats field.\nHowever, this is confusing and error prone as fq_codel_dequeue could also\npotentially mutate this field (which qdisc_dequeue_internal calls in the\nnon gso_skb case), so we have unified the code here with other qdiscs."}],"affected":[{"product":"Linux","vendor":"Linux","defaultStatus":"unaffected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["include/net/sch_generic.h","net/sched/sch_codel.c","net/sched/sch_fq.c","net/sched/sch_fq_codel.c","net/sched/sch_fq_pie.c","net/sched/sch_hhf.c","net/sched/sch_pie.c"],"versions":[{"version":"4b549a2ef4bef9965d97cbd992ba67930cd3e0fe","lessThan":"a225f44d84b8900d679c5f5a9ea46fe9c0cc7802","status":"affected","versionType":"git"},{"version":"4b549a2ef4bef9965d97cbd992ba67930cd3e0fe","lessThan":"52bf272636bda69587952b35ae97690b8dc89941","status":"affected","versionType":"git"}]},{"product":"Linux","vendor":"Linux","defaultStatus":"affected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["include/net/sch_generic.h","net/sched/sch_codel.c","net/sched/sch_fq.c","net/sched/sch_fq_codel.c","net/sched/sch_fq_pie.c","net/sched/sch_hhf.c","net/sched/sch_pie.c"],"versions":[{"version":"3.5","status":"affected"},{"version":"0","lessThan":"3.5","status":"unaffected","versionType":"semver"},{"version":"6.16.4","lessThanOrEqual":"6.16.*","status":"unaffected","versionType":"semver"},{"version":"6.17","lessThanOrEqual":"*","status":"unaffected","versionType":"original_commit_for_fix"}]}],"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.5","versionEndExcluding":"6.16.4"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.5","versionEndExcluding":"6.17"}]}]}],"references":[{"url":"https://git.kernel.org/stable/c/a225f44d84b8900d679c5f5a9ea46fe9c0cc7802"},{"url":"https://git.kernel.org/stable/c/52bf272636bda69587952b35ae97690b8dc89941"}],"title":"net/sched: Fix backlog accounting in qdisc_dequeue_internal","x_generator":{"engine":"bippy-1.2.0"}}}}