CVE-2025-39723 netfs: Fix unbuffered write error handling

What is CVE-2025-39723?

CVE-2025-39723 is a high-severity vulnerability in Linux kernel, published on 5 Sep 2025. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

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.

Weakness: CWE-787

Which versions are affected by CVE-2025-39723?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2025-39723?

Apply the vendor's patch.

Is CVE-2025-39723 being exploited?

There is no known exploitation of CVE-2025-39723 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2025-39723, DEBIAN-CVE-2025-39723, GHSA-6r55-jr8w-3x3r, ROOT-OS-DEBIAN-13-CVE-2025-39723, UBUNTU-CVE-2025-39723.