CVE-2026-90075 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: fq_codel: clamp default quantum and mtu
fq_codel_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) without clamping. A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return 0x80000000, which overflows the signed flow->deficit to INT_MIN in fq_codel_dequeue(), causing an infinite loop and soft lockup. Emulate fq_codel_change() and constrain to [256, FQ_CODEL_QUANTUM_MAX].
The same unclamped psched_mtu() is assigned to q->cparams.mtu a bit below, and fq_codel_change() never updates it. codel_should_drop() tests "*backlog <= params->mtu"; with mtu == 0x80000000 (~2 GiB) and the default 32 MiB memory_limit, the test is always true, so CoDel is silently and completely disabled (no drops, no ECN). Declare a single clamped mtu and assign both q->quantum and q->cparams.mtu from it, which also removes the double psched_mtu() call.
Conditions to recreate the bug: a device whose MTU (plus hard_header_len) wraps psched_mtu() into the sign bit (e.g. a dummy device with max_mtu == 0 accepting MTU 2147483634). Requires CAP_NET_ADMIN in a user namespace.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel networking subsystem contains a critical logic error within the fq_codel queueing discipline implementation that leads to resource exhaustion and denial of service conditions under specific configuration scenarios. The vulnerability stems from the initialization function fq_codel_init, which assigns the device maximum transmission unit directly to internal scheduling parameters without performing necessary boundary checks or clamping operations. Specifically, the code sets the quantum value using psched_mtu on the associated network device interface. While this approach is generally functional for standard interfaces with typical MTU values such as 1500 bytes, it becomes hazardous when interacting with virtual devices that support exceptionally large maximum transmission units. In particular, dummy network interfaces configured with a max_mtu of zero can accept arbitrary high MTU values up to the integer limit, creating an environment where the resulting psched_mtucalculation exceeds safe bounds for signed 32-bit arithmetic operations used elsewhere in the scheduler logic.
When a device is configured with an extremely large MTU such as two billion one hundred forty-seven million four hundred eighty-three thousand six hundred thirty-four bytes, the psched_mtu function returns a value that corresponds to the maximum positive integer plus additional bits depending on internal scaling factors. This oversized value is then assigned directly to q->quantum and later used in calculations within fq_codel_dequeue. The flow deficit variable is defined as a signed 32-bit integer with a range limited by INT_MAX and INT_MIN. When the quantum derived from the massive MTU interacts with subsequent arithmetic operations involving packet sizes and byte counters, it causes an overflow that wraps the value to INT_MIN. This negative infinity-like state disrupts the dequeue logic which relies on positive deficit values to determine when packets can be transmitted. Consequently, the scheduler enters an infinite loop because the condition for advancing the flow pointer or processing the next packet is never satisfied due to the corrupted arithmetic state.
This technical flaw results in a soft lockup within the kernel network stack, effectively causing a denial of service for any process attempting to utilize that specific queueing discipline on the affected interface. The system may become unresponsive regarding network traffic scheduling on that device, requiring either a reboot or manual intervention by an administrator with appropriate privileges to reset the configuration. Beyond the immediate availability impact, there is also a secondary functional degradation related to congestion control mechanisms. The same unclamped MTU value is assigned to q->cparams.mtu and remains static because fq_codel_change does not update it dynamically based on runtime changes or corrections. Inside codel_should_drop, which determines whether packets should be dropped or marked for Explicit Congestion Notification, the logic compares the current backlog size against this mtu parameter. With an MTU value of two billion one hundred forty-seven million four hundred eighty-three thousand six hundred thirty-four bytes and a default memory limit significantly lower than that threshold, the condition is perpetually true. This causes CoDel to be silently disabled entirely, removing essential congestion control capabilities and allowing bufferbloat or potential further resource exhaustion without any packet dropping intervention.
The exploitation of this vulnerability requires specific conditions involving network namespace isolation and administrative privileges. An attacker must possess CAP_NET_ADMIN capability within a user namespace to create the necessary dummy interface and configure its MTU parameters appropriately. This restriction limits the attack surface primarily to local users who have been granted elevated networking permissions, such as container runtimes or virtualization management tools that allow guest OS configurations of host-like network interfaces. While remote exploitation is not directly possible due to these privilege requirements, the impact on system stability remains severe for any multi-tenant environment where untrusted workloads can manipulate network device properties within their isolated namespaces.
To mitigate this vulnerability, kernel developers have implemented a fix that enforces strict bounds checking during initialization. The solution involves clamping both the quantum and MTU parameters to a safe range defined by constants twenty-five six as the lower bound and FQ_CODEL_QUANTUM_MAX as the upper bound. This ensures that all arithmetic operations involving these values remain within valid signed integer limits, preventing overflow into negative territory. Additionally, the fix consolidates the assignment of both q->quantum and q->cparams.mtu from a single clamped MTU value, eliminating redundant calls to psched_mtu and ensuring consistency between scheduling quantum and congestion control thresholds. System administrators should ensure that their Linux kernels are updated with patches addressing this fq_codel initialization logic error. For environments utilizing containerization or virtual networks, it is advisable to review network namespace configurations and restrict the ability of unprivileged users to set extreme MTU values on dummy interfaces unless absolutely necessary for specific high-performance networking requirements where such large frames are legitimately required and properly managed by updated kernel versions.
From a classification perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound, as the core issue arises from arithmetic operations exceeding the capacity of signed 32-bit integers leading to undefined behavior in control flow logic. It also relates to CWE-787 Out-of-bounds Write if considering potential memory corruption implications although here it manifests primarily as a logical loop and denial of service rather than direct memory overwrite. In terms of attack vectors, this falls under ATT&CK technique T1059 Command and Scripting Interpreter via local privilege escalation or resource manipulation within containerized environments where attackers leverage granted capabilities to destabilize shared kernel resources. The failure to validate input parameters from user-space configurations before using them in critical scheduling loops represents a classic example of insufficient validation leading to availability impacts, emphasizing the need for rigorous bounds checking on all network device attributes that influence internal scheduler state machines.