CVE-2026-97610 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
netfs: Fix uninitialized return value in netfs_unbuffered_write()
If preparation of the first subrequest fails, netfs_unbuffered_write() exits its loop before ret is initialized. The empty-iterator check can do the same.
For synchronous writes, netfs_unbuffered_write_iter_locked() may then return an unrelated error instead of wreq->error. This is reachable through CIFS if cifs_prepare_write() fails to reopen the file or obtain credits.
Initialize ret to 0 so the caller returns wreq->error if no data was written, or the number of bytes already written otherwise.
Found with Clang's -Wconditional-uninitialized.
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Analysis
by VulDB Data Team • 09/25/2026
The Linux kernel network filesystem subsystem contains a critical logic error within the netfs_unbuffered_write function that results in the return of an uninitialized stack variable under specific failure conditions. This vulnerability stems from improper initialization of local variables during the execution flow when input validation or resource preparation fails early in the process. Specifically, if the initial subrequest preparation encounters an error, the control loop exits prematurely without assigning a value to the ret variable that is intended to track the number of bytes written or the final status code. Similarly, checks for empty iterators can also lead to this uninitialized state being returned to the caller. This represents a classic instance where defensive programming practices were not fully applied to all exit paths within a complex asynchronous I/O handling routine.
From a technical perspective, this flaw allows the kernel to return garbage data from the stack as an error code or byte count to user-space applications interacting with network file systems. For synchronous write operations handled by netfs_unbuffered_write_iter_locked, this means that instead of returning the specific error encountered during preparation, such as cifs_prepare_write failing due to inability to reopen a file or obtain necessary credits, the function may return an unrelated and potentially misleading error value. This unpredictability complicates debugging for system administrators and developers while introducing potential instability in applications relying on precise error codes from network filesystem operations. The issue was identified using Clang's static analysis tool -Wconditional-uninitialized, which flags variables that are used before being definitively initialized in all code paths.
The operational impact of this vulnerability is primarily related to reliability and security through information leakage or logic bypass. When an application receives a spurious error code due to uninitialized memory contents, it may behave incorrectly, such as retrying operations unnecessarily, failing silently when it should report an error, or crashing if the garbage value interprets as a valid but unexpected state. In the context of CIFS and other network filesystems that rely on this subsystem, users might experience intermittent failures during file writes where the root cause is obscured by the misleading return values. Furthermore, returning uninitialized stack memory can theoretically leak kernel stack contents to user space if the error code interpretation allows for partial data exposure, although in this specific case, it primarily affects control flow and error handling logic rather than direct sensitive data disclosure.
To mitigate this vulnerability, developers must ensure that all local variables used for return values are initialized before any conditional exit points within the function. The primary fix involves initializing the ret variable to zero at the start of netfs_unbuffered_write so that if no data is written due to preparation failures, a clean and predictable value is returned rather than stack garbage. This ensures that wreq->error is properly propagated when appropriate or that zero bytes are correctly reported as written in edge cases. Static analysis tools like Clang's -Wconditional-uninitialized should be integrated into the continuous integration pipeline for kernel development to catch such issues early. Additionally, code reviews should focus on all exit paths within complex loops and conditional blocks to guarantee variable initialization consistency across different execution scenarios involving network filesystem operations.
This vulnerability aligns with CWE-457, which describes the use of an uninitialized variable as a security weakness. It also relates to CWE-252 regarding unchecked return values if subsequent code fails to validate the integrity of the returned status before proceeding. In terms of MITRE ATT&CK mapping, this type of flaw can be leveraged in conjunction with other vulnerabilities for privilege escalation or denial of service attacks by manipulating error handling logic within kernel-space network drivers and filesystem implementations. Proper initialization practices are essential to maintain the integrity of system calls and prevent exploitation through malformed inputs that trigger these specific failure paths during file preparation phases.