CVE-2026-90182 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
blk-iocost: clear delay state when freeing policy data
iocg_kick_delay() turns sufficiently large debt into an explicit block-cgroup delay with blkcg_set_delay(), setting blkg->use_delay to -1 and incrementing blkcg->congestion_count. Clearing it again depends on iocg_kick_delay() running from the period timer, the waitq timer or the issue path.
ioc_pd_free() removes the iocg from active_iocgs and cancels its waitq timer, and no further bios can arrive, so once it has run nothing is left which can reduce the debt and clear the delay. The blkcg stays marked congested for the rest of its life.
blk_cgroup_congested() then returns true for every task in that cgroup and its descendants: page_cache_sync_ra() cuts readahead to a single page, page_cache_async_ra() skips it altogether, and __folio_throttle_swaprate() takes swap_avail_lock and schedules a throttle on anonymous folio allocation.
Clear it explicitly, after the list removal and the synchronous hrtimer_cancel() so that neither timer processing nor an I/O path can re-arm it. The free callback can also see policy data which was never attached to a blkg, hence the pd->blkg check.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel vulnerability identified in the blk-iocost subsystem involves a state management flaw within the block cgroup I/O cost controller that results in persistent congestion signaling and degraded performance for affected control groups. The core issue stems from an asymmetry between how delay states are established and how they are cleared during resource deallocation. Specifically, when a block group incurs significant debt through excessive I/O operations, the iocg_kick_delay function converts this implicit cost into an explicit block-cgroup delay by invoking blkcg_set_delay. This action sets the blkg->use_delay flag to -1 and increments the blkcg->congestion_count, effectively marking the cgroup as congested to throttle future I/O requests. Under normal operational conditions, these delays are cleared when iocg_kick_delay is executed again via periodic timers or during subsequent issue paths that reduce the accumulated debt.
The vulnerability manifests in the ioc_pd_free function, which is responsible for cleaning up policy data associated with a block group. During this cleanup process, the code removes the ioc_group from the active_iocgs list and cancels its wait queue timer using synchronous hrtimer cancellation. Crucially, once these actions are completed, no further I/O bios can arrive to trigger the logic that would normally reduce debt and clear the delay state. Consequently, if a block group is freed while it still holds significant debt, the explicit delay remains set indefinitely because there is no remaining mechanism within the lifecycle of that cgroup instance to reset blkg->use_delay or decrement blkcg->congestion_count. This leaves the block control group permanently marked as congested for its entire lifetime, even after the associated resources have been released.
The operational impact of this persistent congestion state is severe and cascading throughout the kernel's I/O scheduling subsystem. The function blk_cgroup_congested returns true for every task within the affected cgroup and all its descendant groups, triggering aggressive throttling mechanisms designed to alleviate load on a congested block device. For read operations, page_cache_sync_ra reduces the readahead window to a single page, drastically lowering sequential read throughput. Similarly, page_cache_async_ra skips asynchronous readahead entirely, preventing pre-fetching of data that could improve performance for future accesses. Furthermore, swap activity is impacted as __folio_throttle_swaprate acquires the swap_avail_lock and schedules throttling on anonymous folio allocation, which can lead to significant latency spikes in memory swapping operations. These effects collectively result in a denial-of-service condition characterized by severe I/O starvation and system-wide performance degradation for processes operating within the compromised cgroup hierarchy.
To mitigate this vulnerability, explicit clearing of the delay state is required during the cleanup phase of policy data deallocation. The fix involves ensuring that blkg->use_delay is reset to zero after the ioc_group has been removed from the active list and its timers have been synchronously cancelled. This sequence guarantees that neither timer processing nor any residual I/O path can re-arm or maintain the delay state once the resource is being freed. Additionally, because the free callback may encounter policy data structures that were never successfully attached to a block group key structure (blkg), the implementation must include a safety check for pd->blkg before attempting to access and modify blkg-specific fields. This defensive programming approach prevents potential null pointer dereferences while ensuring state consistency across all code paths involved in resource teardown, thereby restoring normal I/O scheduling behavior and preventing persistent congestion artifacts from lingering in the kernel memory space.