CVE-2015-8785

What it actually does

The fuse_fill_write_pages function in fs/fuse/file.c allows a local user to trigger an infinite loop by issuing a writev system call whose first iov segment has zero length, resulting in denial of service.

Why it matters to the business

A local process can render affected systems unresponsive, interrupting workloads and requiring manual intervention or reboot to restore service; older systems that remain unpatched continue to carry this availability risk years after disclosure.

How an attack unfolds

An attacker with local access on a system using FUSE mounts crafts a writev call that supplies a zero-length first segment, causing the kernel loop in fuse_fill_write_pages to execute indefinitely and consume CPU resources until the system is rebooted.

How to fix it

Upgrade the Linux kernel to version 4.4 or later on all affected distributions; apply vendor-supplied packages that contain the corrected fuse_fill_write_pages logic; where immediate patching is not feasible, restrict untrusted local accounts and limit FUSE usage to trusted workloads.

How to confirm the fix worked

Confirm the running kernel is 4.4 or newer with uname -r; review package manager history for the applied security update; attempt to reproduce the zero-length iov writev condition in a controlled test environment and verify the loop no longer occurs.

Typical attack path

1. Obtain local shell or process execution on target host
2. Locate or create a FUSE-mounted filesystem
3. Invoke writev system call with first iov segment length set to zero
4. Observe kernel thread enter infinite loop and system resources become exhausted

How to detect it

Review system and audit logs for repeated writev calls from untrusted processes; monitor for sustained high CPU usage in kernel threads associated with FUSE without corresponding user activity; examine crash dumps or hung-task reports that reference fuse_fill_write_pages.

What your security team should do

• Prioritize kernel upgrades on all internet-facing or multi-tenant systems within the next maintenance window
• Add kernel version checks to existing vulnerability scanning and asset inventory processes
• Restrict FUSE filesystem mounts to administrative users until patches are deployed