Linux Kernel up to 6.12.9/6.13-rc6 netfs_extract_user_iter buffer overflow

Summaryinfo

A vulnerability was found in Linux Kernel up to 6.12.9/6.13-rc6. It has been declared as critical. This affects the function netfs_extract_user_iter. Executing a manipulation can lead to buffer overflow. The identification of this vulnerability is CVE-2025-21643. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability classified as critical was found in Linux Kernel up to 6.12.9/6.13-rc6. This vulnerability affects the function netfs_extract_user_iter. The manipulation with an unknown input leads to a buffer overflow vulnerability. The CWE definition for the vulnerability is CWE-120. The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

In the Linux kernel, the following vulnerability has been resolved: netfs: Fix kernel async DIO Netfslib needs to be able to handle kernel-initiated asynchronous DIO that is supplied with a bio_vec[] array. Currently, because of the async flag, this gets passed to netfs_extract_user_iter() which throws a warning and fails because it only handles IOVEC and UBUF iterators. This can be triggered through a combination of cifs and a loopback blockdev with something like: mount //my/cifs/share /foo dd if=/dev/zero of=/foo/m0 bs=4K count=1K losetup --sector-size 4096 --direct-io=on /dev/loop2046 /foo/m0 echo hello >/dev/loop2046 This causes the following to appear in syslog: WARNING: CPU: 2 PID: 109 at fs/netfs/iterator.c:50 netfs_extract_user_iter+0x170/0x250 [netfs] and the write to fail. Fix this by removing the check in netfs_unbuffered_write_iter_locked() that causes async kernel DIO writes to be handled as userspace writes. Note that this change relies on the kernel caller maintaining the existence of the bio_vec array (or kvec[] or folio_queue) until the op is complete.

The advisory is shared for download at git.kernel.org. This vulnerability was named CVE-2025-21643 since 12/29/2024. The exploitation appears to be easy. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 10/17/2025).

The vulnerability scanner Nessus provides a plugin with the ID 233479 (Ubuntu 24.10 : Linux kernel (Low Latency) vulnerabilities (USN-7380-1)), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.12.10 or 6.13-rc7 eliminates this vulnerability. Applying the patch 9f3a265836844eda30bf34c2584b8011fd4f0f49/3f6bc9e3ab9b127171d39f9ac6eca1abb693b731 is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (233479), EUVD (EUVD-2025-2592) and CERT Bund (WID-SEC-2025-0119). Once again VulDB remains the best source for vulnerability data.

Affected

• Debian Linux
• Amazon Linux 2
• Red Hat Enterprise Linux
• Ubuntu Linux
• SUSE Linux
• Oracle Linux
• Dell NetWorker
• Dell Avamar
• Dell PowerProtect Data Domain
• Open Source Linux Kernel

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 6.12.0
• 6.12.1
• 6.12.2
• 6.12.3
• 6.12.4
• 6.12.5
• 6.12.6
• 6.12.7
• 6.12.8
• 6.12.9
• 6.13-rc1
• 6.13-rc2
• 6.13-rc3
• 6.13-rc4
• 6.13-rc5
• 6.13-rc6

License

• open-source

Website

• Vendor: https://www.kernel.org/

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