CVE-2026-53191 Overview
CVE-2026-53191 is a Linux kernel vulnerability in the io_uring/net subsystem affecting bundle receive retry logic. The flaw occurs when io_recv_finish() merges completion queue entry (CQE) flags across retry iterations. The IORING_CQE_F_BUF_MORE flag was excluded from the CQE_F_MASK, causing the merge logic to drop it when the final retry iteration partially consumed a provided buffer ring entry. Userspace consumers can then advance the ring head past a buffer the kernel still references, producing buffer state desynchronization between kernel and user space.
Critical Impact
Userspace can wrongfully advance the provided buffer ring head past entries the kernel is still using, leading to buffer reuse inconsistencies in io_uring network workloads.
Affected Products
- Linux kernel io_uring/net subsystem with provided buffer rings (IOU_PBUF_RING_INC) in incremental mode
- Linux kernel versions supporting bundle receive operations with io_uring
- Stable kernel branches receiving the referenced commits (0bbc9481, 4973232a, ed46f39c, f40570fd)
Discovery Timeline
- 2026-06-25 - CVE-2026-53191 published to NVD
- 2026-06-25 - Last updated in NVD database
Technical Details for CVE-2026-53191
Vulnerability Analysis
The vulnerability resides in the bundle receive retry path of io_uring/net. When io_recv_finish() processes a retry, it merges the saved cflags from the prior iteration with cflags returned by the new iteration using the expression cflags = req->cqe.flags | (cflags & CQE_F_MASK);. Bits listed in CQE_F_MASK are inherited from the new iteration, while remaining bits, including IORING_CQE_F_BUFFER and the buffer ID, are carried from the saved value.
Before the fix, CQE_F_MASK covered only IORING_CQE_F_SOCK_NONEMPTY and IORING_CQE_F_MORE. The IORING_CQE_F_BUF_MORE flag, set by __io_put_kbufs() when io_kbuf_inc_commit() leaves a head ring entry partially consumed, was silently dropped during the merge.
Root Cause
The root cause is an incomplete mask definition. CQE_F_MASK did not include IORING_CQE_F_BUF_MORE, so the OR-merge logic preserved stale carry-over state and discarded the indicator that a provided buffer would be reused in subsequent completions. The subsequent req->cqe.flags = cflags & ~CQE_F_MASK save could also retain stale IORING_CQE_F_BUF_MORE bits between iterations.
Attack Vector
The issue is triggered through legitimate io_uring usage patterns: bundle recv operations combined with provided buffer rings in incremental mode. When the final retry iteration partially consumes a buffer, userspace receives a CQE without the IORING_CQE_F_BUF_MORE indicator. The userspace consumer then treats the buffer as fully consumed and advances the ring head, while the kernel continues to reference the same buffer ID for upcoming completions. This produces buffer state desynchronization that can corrupt received network data or trigger incorrect buffer recycling.
The vulnerability is described in kernel commits and does not require crafted exploitation code. See the kernel commit f40570fd for the corrected mask logic.
Detection Methods for CVE-2026-53191
Indicators of Compromise
- Anomalous io_uring completion queue entries missing expected IORING_CQE_F_BUF_MORE flags when using provided buffer rings in incremental mode
- Network application errors involving corrupted or misaligned receive buffers under workloads using bundle recv
- Kernel logs referencing io_uring/net buffer accounting inconsistencies
Detection Strategies
- Audit running kernel versions against the patched commits (0bbc9481, 4973232a, ed46f39c, f40570fd) to identify unpatched hosts
- Inventory applications using io_uring with provided buffer rings and bundle recv, focusing on high-throughput network services
- Review application-level error reporting for buffer reuse anomalies in io_uring-based network stacks
Monitoring Recommendations
- Track kernel version drift across the fleet and flag hosts running pre-patch io_uring/net code paths
- Monitor user-space services consuming io_uring CQEs for unexpected buffer accounting errors or crashes
- Correlate workload changes with kernel patch deployment windows to validate remediation
How to Mitigate CVE-2026-53191
Immediate Actions Required
- Apply the upstream Linux kernel patches referenced in the stable tree commits 0bbc9481, 4973232a, ed46f39c, and f40570fd
- Update to vendor-supplied kernel packages that include the corrected CQE_F_MASK definition
- Reboot affected systems after kernel upgrade to load the patched io_uring/net code
Patch Information
The fix adds IORING_CQE_F_BUF_MORE to CQE_F_MASK so that the flag is inherited from the new iteration into the user-visible CQE and stripped from the saved cflags between iterations. Reference patches: Kernel Git Commit 0bbc9481, Kernel Git Commit 4973232a, Kernel Git Commit ed46f39c, and Kernel Git Commit f40570fd.
Workarounds
- Disable use of provided buffer rings in incremental mode (IOU_PBUF_RING_INC) combined with bundle recv until patching is complete
- Restrict io_uring usage in untrusted workloads via seccomp filters or kernel configuration where feasible
- Validate userspace consumers do not assume IORING_CQE_F_BUF_MORE semantics on unpatched kernels
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
