CVE-2026-98797 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
net: cap skb->queue_mapping when the tx queue is picked
skbedit can set skb->queue_mapping and raise the per-CPU skip_txqueue flag so __dev_queue_xmit() honours the mapping. __dev_queue_xmit() cleared the flag before sch_handle_egress() and only read it afterwards, so the flag was not confined to the xmit that set it: a nested xmit (mirred redirect or mirror, or a drop after skbedit) could set the flag and the outer xmit would consume it for an skb that never went through skbedit.
A forwarded packet still carries the ingress NIC's rx_queue + 1 in skb->queue_mapping, so the outer device then indexes its tx queue state with that stale value. Taprio's child array q->qdiscs[] is sized to the
device's queue count, so taprio_enqueue() indexes past its allocation and dereferences the result as a struct Qdisc *.
We (ab)use the skb->nf_skip_egress which means "skip netfilter egress for this packet" to tag to "am I in tc egress?". Despite the overload I dont see it as a conflict since the marker is set only around the single sch_handle_egress() call and ingress path is guarded by tc_at_ingress. I will send a followup(net-next) patch once this hits net-next to rename the skb->nf_skip_egress bit/flag to skb->skip_egress
Arm the flag only from the egress classifier that can use it: raise skip_txqueue from tcf_skbedit_act() only when it runs inside sch_handle_egress(), thanks to skb->nf_skip_egress. An egress qdisc classifier runs in q->enqueue(), after the tx queue has been picked, so a mapping it sets cannot affect the current packet; arming the flag there only pollutes it for a later xmit. Then own the flag for the xmit frame the egress hook runs in: save the incoming value and clear it just before sch_handle_egress(), and restore it after the hook - on the consumed (drop) path, or, in the same call that reads it, on the surviving path. The save and the restores stay inside the egress_needed_key static branch, so a packet pays for them only when egress hooks are active (2f1e85b1aee4).
Store the value netdev_cap_txqueue() selected back into skb->queue_mapping in netdev_tx_queue_mapping(), as netdev_core_pick_tx() already does, so the skip_txqueue path never hands a later reader on the xmit path a mapping the device cannot serve. A store made still later in the same frame, by a tc BPF program attached to a transmit qdisc, is outside this path and is not re-capped; a separate followup will resolve that path.
netdev_xmit_skip_txqueue() returns the previous flag value so the save-and-clear is one call, and a no-op stub is provided when CONFIG_NET_EGRESS is disabled. skb->nf_skip_egress is compiled under CONFIG_NET_EGRESS rather than CONFIG_NETFILTER_SKIP_EGRESS, so skb_at_tc_egress() is valid whenever the egress path is built.
A local user in a network namespace can redirect a packet from a device with more TX queues to one with fewer after setting a mapping valid only on the larger device. That reaches these reads and, under KASAN, faults with "slab-out-of-bounds in taprio_enqueue".
Conditions to recreate the bug: the report's own trigger is a local user with CAP_NET_ADMIN in a network namespace, so no eBPF program is needed. With CONFIG_NET_SCH_TAPRIO=y, CONFIG_NET_ACT_SKBEDIT=y, CONFIG_NET_ACT_MIRRED=y, CONFIG_NET_CLS_MATCHALL=y, CONFIG_NET_SCH_PRIO=y and KASAN enabled, create qa (3 queues), qb (2 queues) and qc (1 queue) as dummy devices; put a taprio root on qb (num_tc 1, queues 2@0) and clsact on all three; then add an egress matchall filter on every device. On qa: "action skbedit queue_mapping 2 pipe action mirred egress redirect dev qb". On qb: "action mirred egress mirror dev qc". On qc: "action skbedit queue_mapping 0 pipe". Send one packet out qa. qc's skbedit sets the flag while qb's outer xmit is in flight; without the fix qb consumes it and reads its two-entry taprio child array with the forwarded packet's stale mapping. A qc whose skb ---truncated---
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Analysis
by VulDB Data Team • 10/18/2026
The vulnerability identified as CVE-2024-10857 represents a critical out-of-bounds memory access within the Linux kernel networking subsystem, specifically affecting traffic control mechanisms involving queue mapping and egress scheduling. The flaw resides in the interaction between socket buffer management functions and device transmission queues during nested packet processing operations such as mirroring or redirection. A local user possessing CAP_NET_ADMIN privileges within a network namespace can exploit this condition to trigger a slab-out-of-bounds error, which may lead to kernel panic or potential privilege escalation depending on memory layout conditions. The issue stems from improper handling of the skip_txqueue flag and queue mapping values across nested transmission contexts, allowing stale data from one device context to corrupt operations in another with different hardware constraints.
The technical root cause involves a race condition-like logic error in how the kernel handles packet redirection between network devices with differing numbers of transmit queues. When an egress classifier action such as skbedit sets a queue mapping on a socket buffer, it also raises a per-CPU skip_txqueue flag to ensure __dev_queue_xmit honors this specific mapping. However, prior to the fix, __dev_queue_xmit cleared this flag before calling sch_handle_egress and only read it afterwards. This sequence allowed nested transmission events, such as those triggered by mirred redirect or mirror actions, to set the skip_txqueue flag while an outer transmission was already in progress. Consequently, the outer xmit operation would consume a flag that belonged to a different packet context, leading to incorrect queue index calculations for subsequent operations.
This logic error becomes particularly dangerous when interacting with complex traffic control schedulers like Taprio. A forwarded packet retains the ingress NIC's rx_queue value plus one within its skb->queue_mapping field. When this stale mapping is applied to an outer device that has fewer transmit queues than the original source, the resulting index exceeds the bounds of the scheduler's internal child array q->qdiscs[]. Since taprio_enqueue dereferences this out-of-bounds index as a struct Qdisc pointer, it results in invalid memory access. The vulnerability effectively allows a user to manipulate kernel heap structures by forcing the kernel to read from or write to memory locations outside the allocated slab for queue discipline objects.
The exploitation scenario requires specific configuration conditions including CONFIG_NET_SCH_TAPRIO and CONFIG_NET_ACT_SKBEDIT enabled alongside KASAN debugging features which make the out-of-bounds access immediately fatal via a fault. An attacker creates dummy network devices with varying numbers of transmit queues, attaches Taprio to one device and clsact filters to all involved interfaces. By configuring an egress matchall filter that redirects packets from a multi-queue device to a single-queue device using skbedit and mirred actions, the attacker forces the kernel into the vulnerable state where stale queue mappings are applied incorrectly during nested transmission processing. This triggers the slab-out-of-bounds error in taprio_enqueue due to invalid array indexing based on corrupted queue mapping data.
Mitigation strategies primarily involve applying vendor-provided kernel patches that address this specific logic flaw in network stack handling. The fix involves restructuring how the skip_txqueue flag is managed by ensuring it is only armed from egress classifiers when running inside sch_handle_egress, thereby confining its scope to the correct transmission frame. Additionally, the implementation saves and restores queue mapping values correctly around egress hooks to prevent stale data propagation between nested xmit operations. System administrators should ensure their kernels are updated with these security fixes, particularly if they utilize advanced traffic control features involving multiple network interfaces with different hardware capabilities or employ complex redirection rules in containerized environments where namespace isolation might be leveraged for such attacks.
From a classification perspective, this vulnerability aligns with CWE-125 Out-of-bounds Read and CWE-362 Race Condition within the context of kernel state management. It also maps to MITRE ATT&CK techniques related to privilege escalation via exploitation of local vulnerabilities, specifically leveraging improper input validation in system components that handle network packet processing. The incident highlights the risks associated with complex software-defined networking features where multiple layers of abstraction interact without sufficient boundary checks on resource indices derived from user-controlled parameters like queue mappings.