CVE-2025-39691: Linux Kernel Use-After-Free Vulnerability

CVE-2025-39691 Overview

CVE-2025-39691 is a use-after-free vulnerability [CWE-416] in the Linux kernel's fs/buffer subsystem. The flaw occurs in the bh_read() helper when processing buffer heads allocated on the stack, specifically during NTFS3 filesystem mount operations. When unlock_buffer() unlocks and wait_on_buffer() returns to the caller, the stack-allocated map_bh variable can be reclaimed before end_buffer_read_sync() completes. The subsequent put_bh() call then dereferences reclaimed stack memory, triggering a stack-out-of-bounds condition detected by KASAN. The vulnerability affects Linux kernel versions from 2.6.12 through 6.17-rc2 and Debian Linux 11.0.

Critical Impact
A local authenticated attacker can trigger kernel memory corruption leading to system crash, denial of service, or potential privilege escalation through carefully timed I/O completion races.

Affected Products

Linux Kernel (multiple versions from 2.6.12 through 6.17-rc2)
Debian Linux 11.0
Siemens products referenced in advisories ssa-019113 and ssa-032379

Discovery Timeline

2025-09-05 - CVE-2025-39691 published to the National Vulnerability Database (NVD)
2026-07-30 - Last updated in NVD database

Technical Details for CVE-2025-39691

Vulnerability Analysis

The defect resides in the Linux kernel buffer head handling code path invoked during NTFS3 filesystem mount. The mpage_read_folio() function passes a stack-allocated map_bh variable to ntfs_get_block_vbo(), which then calls bh_read(). Inside bh_read(), the code calls submit_bh() followed by wait_on_buffer(bh).

When a block I/O completion interrupt fires, the softirq path executes end_bio_bh_io_sync() and then end_buffer_read_sync(). The completion handler invokes __end_buffer_read_notouch(), which calls unlock_buffer(). Once the lock releases, wait_on_buffer() returns in the mount context and the calling function may exit, reclaiming the stack frame containing map_bh.

The softirq continues execution and calls put_bh() on the now-invalidated stack address, producing the KASAN stack-out-of-bounds report at end_buffer_read_sync+0xe3/0x110. Because the offending address lives on another CPU's stack, memory contents at that location are unpredictable.

Root Cause

The root cause is an ordering bug in end_buffer_read_sync(). Buffer heads allocated on the stack become invalid the moment unlock_buffer() releases the waiting thread. However, put_bh() executes after unlock_buffer(), dereferencing memory that has already been reclaimed. The fix reorders the operations so put_bh() runs before __end_buffer_read_notouch() when safe to do so.

Attack Vector

Exploitation requires local access with privileges sufficient to trigger filesystem mount operations or block-device I/O against NTFS3 or similar callers of bh_read() with stack-based buffer heads. An attacker who can influence mount timing and I/O completion could race the kernel into dereferencing arbitrary stack contents. The window is narrow and depends on scheduler timing, but reliable triggering causes kernel panic and can potentially be shaped into memory corruption primitives.

No public proof-of-concept exploit is currently available for CVE-2025-39691. See the Linux kernel commit fix for technical details of the resolution.

Detection Methods for CVE-2025-39691

Indicators of Compromise

Kernel oops or panic entries in dmesg or /var/log/kern.log referencing end_buffer_read_sync, end_bio_bh_io_sync, or bh_read
KASAN reports containing stack-out-of-bounds in end_buffer_read_sync when running debug kernels
Unexpected system reboots or crashes correlated with NTFS3 mount operations or removable-media insertion

Detection Strategies

Enable CONFIG_KASAN on non-production or canary systems to surface the exact fault signature during QA and pre-production testing
Audit kernel version inventory against the patched stable trees listed in the kernel.org advisories to identify unpatched hosts
Correlate filesystem mount events (auditdMOUNT records) with subsequent kernel messages indicating memory faults

Monitoring Recommendations

Ingest /var/log/kern.log, journald kernel entries, and auditd mount events into a centralized log platform for correlation
Alert on kernel stack traces containing end_buffer_read_sync or KASAN reports referencing buffer head functions
Track mount operations of untrusted filesystems (NTFS3, exFAT, and other stack-bh_read callers) on servers where such filesystems are not expected

How to Mitigate CVE-2025-39691

Immediate Actions Required

Apply the upstream Linux kernel patch from the stable trees referenced in the kernel.org commits or update to a kernel version that includes the fix
Debian 11 administrators should install the fixed packages from the Debian LTS Announcement
Siemens customers should review the Siemens Security Advisory SSA-019113 and SSA-032379 for product-specific guidance
Restrict mount privileges to trusted administrative accounts and disable automatic mounting of untrusted removable media

Patch Information

The fix is distributed across multiple stable kernel branches. Patched commits include 03b40bf5d038, 042cf48ecf67, 3169edb8945c, 70a09115da58, 7375f22495e7, 90b5193edb32, c58c6b532b7b, and c5aa6ba11273. The remediation reorders put_bh() to execute before __end_buffer_read_notouch() when the buffer head belongs to a folio, which is safe because drop_buffers() will not free buffers that remain locked. Vendor kernels from Debian and Siemens have shipped corresponding updates.

Workarounds

Disable or blacklist the NTFS3 kernel module (modprobe -r ntfs3; add blacklist ntfs3 to /etc/modprobe.d/) where NTFS support is not required
Prohibit non-root mounting through udev and polkit rules until the patched kernel is deployed
Isolate systems that must mount untrusted filesystems into segmented environments with reduced blast radius

bash

# Verify installed kernel version and apply update
uname -r

# Debian / Ubuntu: install patched kernel
sudo apt update && sudo apt install --only-upgrade linux-image-$(uname -r | sed 's/[^-]*-[^-]*-//')
sudo reboot

# Optional: blacklist ntfs3 until patched kernel is running
echo "blacklist ntfs3" | sudo tee /etc/modprobe.d/blacklist-ntfs3.conf
sudo update-initramfs -u