CVE-2026-90323 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
ublk: validate auto buf reg before taking uring_cmd
With UBLK_F_AUTO_BUF_REG, invalid sqe->addr can fail after ublk_fill_io_cmd() has set UBLK_IO_FLAG_ACTIVE. The uring_cmd is completed while the tag stays active, which can hang teardown.
Split validation from buffer apply so the check has no side effects, then take the uring_cmd and store the already-validated buffer. Apply the same order in FETCH so io->buf is not written before __ublk_fetch() state checks.
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Analysis
by VulDB Data Team • 09/18/2026
The Linux kernel's ublk subsystem contains a race condition vulnerability related to asynchronous I/O command processing that can lead to system hangs during teardown operations. This issue specifically affects configurations utilizing the UBLK_F_AUTO_BUF_REG feature, which allows for automatic buffer registration within the io_uring interface. The core of the problem lies in the ordering of validation and state management steps when handling submission queue entries. Specifically, invalid user-space addresses provided in sqe->addr can cause failures after ublk_fill_io_cmd() has already set the UBLK_IO_FLAG_ACTIVE flag on an internal command structure. This premature activation creates a dangerous state where the io_uring command is marked as completed and returned to the application layer while the underlying tag remains active within the kernel's tracking mechanisms.
This inconsistency between the completion status reported to userspace and the actual lifecycle state of the resource in kernelspace results in a deadlock scenario during device teardown. When the system attempts to shut down or reset the ublk device, it expects all active tags to be properly released. However, because the tag was left in an active state despite the command failing due to invalid input validation occurring too late in the process, the cleanup routine waits indefinitely for a release signal that will never arrive. This effectively hangs the teardown process, potentially causing denial of service conditions or requiring manual intervention such as killing processes or rebooting the system to restore normal operation.
The technical flaw is rooted in improper separation of concerns within the command handling logic. The original implementation performed buffer validation and application concurrently with state flag updates, allowing side effects from failed validations to persist in the active state machine. By splitting the validation step from the buffer application step, the fix ensures that checks for invalid addresses are executed before any flags indicating activity are set. This guarantees that if a submission queue entry contains an invalid address, the command is rejected and completed without ever entering the ACTIVE state, thereby preventing orphaned tags from persisting in memory structures during subsequent teardown sequences.
The remediation strategy involves reordering operations to enforce strict preconditions for state transitions. Validation of sqe->addr must occur before taking the io_uring command or storing buffer references. This approach ensures that only fully validated and safe buffers are associated with active commands. The same logical correction is applied to FETCH operations, ensuring that internal buffer pointers like io->buf are not written until after __ublk_fetch() has completed its state checks. This prevents partial writes into data structures when underlying conditions for successful operation have not been met, maintaining consistency between the kernel's view of resource allocation and userspace expectations.
From a vulnerability classification perspective, this issue aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as it involves race-like behavior in state management leading to inconsistent object lifecycles. It also relates to CWE-824: Access of Uninitialized Pointer and CWE-190: Integer Overflow or Wraparound if the invalid address check fails due to improper bounds checking logic prior to flag setting. In terms of MITRE ATT&CK, this vulnerability could be leveraged in a Denial of Service context by an attacker who can submit crafted io_uring commands with maliciously constructed submission queue entries containing invalid addresses. While typically requiring local access and specific privileges related to ublk device interaction, the ability to hang system teardown processes impacts availability, which is a critical component of security triage.
Mitigation for this vulnerability requires applying kernel patches that enforce the corrected validation order in both UBLK_F_AUTO_BUF_REG handling paths and FETCH operations. System administrators should ensure their Linux kernels are updated to versions where these ordering fixes have been merged into stable releases. For environments unable to immediately patch, restricting access to ublk devices to trusted users can reduce exposure, although this is a less effective control given the local nature of the exploit vector. Continuous monitoring for unusual hangs during device removal or system shutdown events may help detect exploitation attempts in real-time until patches are deployed across all affected systems.