CVE-2026-99264 in Linux
Summary
by MITRE • 11/03/2026
In the Linux kernel, the following vulnerability has been resolved:
io_uring/net: don't retry recv on uncommitted buffers
Commit 41b70df5b38b ("io_uring/net: commit partial buffers on retry") commits ring provided buffers when a receive is going to be retried after a partial transfer, so that another request using the same buffer group can't be handed the memory the first one keeps receiving into. It commits the number of bytes received so far. That does the job for a single buffer which is consumed as a whole, but the request hangs on to all it has mapped:
- For a buffer ring with IOU_PBUF_RING_INC only these bytes are consumed. The rest of the buffer stays at the head of the ring. - For a bundle only the buffers which hold these bytes are consumed, the others that were peeked stay in the ring.
In both cases the next receive with that buffer group gets memory that the retried request is still going to write to, and whoever comes last overwrites the data of the other. What the retry fills isn't committed at all, as the buffer list isn't known any more at that point. So the kernel hands it out again later on, while the CQE tells the application that it was used, and for an incremental buffer that it is done with.
The remainder can't be committed in advance: if the request ends up receiving less, there is no way to tell the application that more was consumed than the CQE accounts for. Complete the request with what it has got instead of retrying it. MSG_WAITALL is no longer honoured for receives into incremental buffers or multi buffer bundles, they can return a short count, with the request marked as failed like for other short MSG_WAITALL transfers.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability described involves a race condition and data integrity issue within the Linux kernel's io_uring networking subsystem, specifically concerning how receive operations handle partial buffers in buffer rings or bundles. The core technical flaw stems from an inconsistency between memory commitment logic and actual usage tracking during retry scenarios. When a receive operation is interrupted by conditions such as EAGAIN and requires a retry after transferring only part of the data, the kernel previously attempted to commit the partially received bytes to prevent other requests from accessing that same buffer space prematurely. However, this approach failed to account for the fact that the original request retained ownership or mapping rights over the entire allocated buffer region rather than just the committed portion. This discrepancy creates a window where subsequent receive operations targeting the same buffer group are handed memory regions that remain actively mapped and writable by the pending retry operation of the first request.
This architectural oversight leads to severe data corruption risks, as multiple concurrent requests can write into overlapping or identical memory locations simultaneously. In scenarios involving incremental buffer rings with IOU_PBUF_RING_INC semantics, only the specific bytes received are marked as consumed, leaving the remainder of the buffer at the head of the ring available for reuse by other processes while still being written to by the original request. Similarly, in multi-buffer bundles, buffers that were merely peeked but not fully utilized remain in the ring despite being subject to ongoing write operations from the retried request. Consequently, whichever process completes its operation last overwrites data intended for another, resulting in unpredictable application behavior and potential information disclosure or corruption depending on the context of the affected applications.
From a security perspective, this flaw aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as it involves race conditions arising from improper handling of shared memory resources across multiple kernel requests. The lack of proper synchronization between buffer commitment and actual usage tracking allows for data races that compromise the integrity of user-space applications relying on io_uring for high-performance I/O operations. Furthermore, this issue can be mapped to MITRE ATT&CK techniques related to resource manipulation or privilege escalation if an attacker can exploit the race condition to overwrite critical application memory structures, potentially leading to arbitrary code execution in contexts where kernel-supplied buffers are used for sensitive data processing.
The operational impact of this vulnerability includes potential denial of service through application crashes caused by corrupted data states, as well as confidentiality breaches if sensitive information is overwritten or exposed via shared buffer regions. Applications utilizing incremental buffering strategies or multi-buffer bundles are particularly susceptible to these issues because they rely on precise tracking of which portions of a buffer have been successfully committed and consumed. The failure to correctly manage the lifecycle of uncommitted but mapped memory segments undermines the reliability guarantees provided by io_uring, forcing applications to handle unexpected data corruption that was previously assumed impossible under normal operation conditions.
To mitigate this vulnerability, the kernel developers implemented a fix that alters the retry logic for receive operations into incremental buffers or multi-buffer bundles. Instead of attempting to commit partial bytes and then risking further conflicts during retries, the system now completes the request with whatever amount of data has been successfully received so far. This approach ensures that buffer ownership is clearly relinquished upon completion, preventing subsequent requests from accessing memory still in use by previous operations. Notably, this change means that MSG_WAITALL semantics are no longer honored for these specific types of buffers; if a receive operation cannot fulfill the full requested amount due to partial transfers and retries, it returns with a short count and marks the request as failed rather than retrying indefinitely or leaving memory in an ambiguous state. This trade-off prioritizes data integrity and synchronization correctness over strict adherence to wait-all semantics for these complex buffer configurations.
Administrators and developers should ensure that their systems are updated with kernel versions containing this fix, identified by commit 41b70df5b38b. For applications relying on io_uring, it is crucial to update codebases to handle short counts and failed requests appropriately when using incremental buffers or multi-buffer bundles, as the behavior regarding MSG_WAITALL has changed. Monitoring for unexpected request failures in high-throughput network services utilizing these specific buffer types can help detect residual issues before they escalate into data corruption events. Regular patching of the Linux kernel is essential to maintain the integrity and security of systems depending on io_uring for efficient I/O operations, particularly those involving complex buffering strategies where precise memory management is critical.