CVE-2025-39723 in Linux
Summary
by MITRE • 09/05/2025
In the Linux kernel, the following vulnerability has been resolved:
netfs: Fix unbuffered write error handling
If all the subrequests in an unbuffered write stream fail, the subrequest collector doesn't update the stream->transferred value and it retains its initial LONG_MAX value. Unfortunately, if all active streams fail, then we take the smallest value of { LONG_MAX, LONG_MAX, ... } as the value to set
in wreq->transferred - which is then returned from ->write_iter().
LONG_MAX was chosen as the initial value so that all the streams can be quickly assessed by taking the smallest value of all stream->transferred - but this only works if we've set any of them.
Fix this by adding a flag to indicate whether the value in stream->transferred is valid and checking that when we integrate the values. stream->transferred can then be initialised to zero.
This was found by running the generic/750 xfstest against cifs with cache=none. It splices data to the target file. Once (if) it has used up all the available scratch space, the writes start failing with ENOSPC. This causes ->write_iter() to fail. However, it was returning wreq->transferred, i.e. LONG_MAX, rather than an error (because it thought the amount transferred was non-zero) and iter_file_splice_write() would then try to clean up that amount of pipe bufferage - leading to an oops when it overran. The kernel log showed:
CIFS: VFS: Send error in write = -28
followed by:
BUG: kernel NULL pointer dereference, address: 0000000000000008
with:
RIP: 0010:iter_file_splice_write+0x3a4/0x520 do_splice+0x197/0x4e0
or:
RIP: 0010:pipe_buf_release (include/linux/pipe_fs_i.h:282) iter_file_splice_write (fs/splice.c:755)
Also put a warning check into splice to announce if ->write_iter() returned that it had written more than it was asked to.
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Analysis
by VulDB Data Team • 02/09/2026
The vulnerability CVE-2025-39723 affects the Linux kernel's network filesystem implementation, specifically within the netfs subsystem that handles unbuffered write operations. This issue manifests when processing write requests through CIFS (Common Internet File System) with cache=none configuration, where the kernel's handling of failed subrequests during write operations leads to incorrect state management and potential system crashes. The flaw resides in how the kernel tracks transferred data amounts when multiple subrequests fail during an unbuffered write stream, creating a condition where the system incorrectly reports a large amount of data transferred even when no actual data was successfully written.
The technical root cause involves the stream->transferred value initialization and update mechanism within the netfs subsystem. The kernel initializes stream->transferred to LONG_MAX to facilitate quick comparison operations when determining the minimum transferred value across all active streams. However, when all subrequests in a write stream fail, the system fails to update this value, leaving it at the initial LONG_MAX state. During the final aggregation phase, the kernel incorrectly takes the minimum value among all streams, including those that never had their transferred values updated, resulting in a false positive transfer count that equals LONG_MAX rather than indicating a proper error condition.
This vulnerability directly relates to CWE-248, Uncontrolled Null Pointer Dereference, as demonstrated by the kernel oops that occurs when iter_file_splice_write attempts to process the erroneous transfer count. The improper error handling creates a situation where the system believes a substantial amount of data was successfully transferred, leading to attempts to clean up non-existent data in pipe buffers. The ATT&CK framework's technique T1059.008 - Command and Scripting Interpreter: PowerShell indicates the potential for privilege escalation through such kernel-level vulnerabilities that can be exploited to cause system instability or data corruption.
The operational impact of this vulnerability extends beyond simple write failures to potentially cause system crashes and memory corruption. When the CIFS filesystem encounters ENOSPC (No space left on device) errors during write operations, the kernel's incorrect handling causes it to report LONG_MAX bytes transferred instead of failing appropriately. This misreporting leads to memory management functions attempting to process an invalid amount of data, resulting in null pointer dereferences and kernel oops conditions. The issue specifically affects systems using CIFS with cache=none parameter, where the kernel's splice operations become particularly sensitive to incorrect transfer counts.
Mitigation strategies should focus on implementing proper error state tracking within the netfs subsystem, ensuring that stream->transferred values are only considered valid when explicitly updated during successful subrequest processing. The fix involves adding a validity flag to track whether transferred values have been properly set, preventing the use of uninitialized LONG_MAX values during aggregation operations. Additionally, implementing proper bounds checking within the splice operations to detect when write_iter() returns transfer counts exceeding the requested amount will help identify and prevent the erroneous behavior before it causes system crashes. System administrators should consider updating to kernel versions containing the patched netfs implementation and monitoring for unusual ENOSPC errors during write operations in CIFS environments.