CVE-2025-68821: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-68821 is a deadlock vulnerability in the Linux kernel's FUSE (Filesystem in Userspace) subsystem, specifically in the readahead reclaim path. The flaw was introduced by commit e26ee4efbc79 ("fuse: allocate ff->release_args only if release is needed"), which skipped grabbing an inode reference in fuse_prepare_release() when the server does not implement open. This omission allows an inode to be evicted from the dcache while inflight readahead requests are still active, creating a deadlock condition when memory reclaim attempts to evict the same inode. The vulnerability affects the Linux kernel and was published on January 13, 2026. No CVSS score has been officially assigned at this time (Red Hat CVE, CVE.org).

The root cause is a missing inode reference count in fuse_prepare_release() when the FUSE server does not implement the open operation (CWE-667: Improper Locking). When a readahead request is in-flight, the folio (memory page) is locked. If the FUSE server triggers memory reclaim during this time, the kernel's dcache shrinker (prune_dcache_sb) may attempt to evict the inode via fuse_evict_inode(), which calls truncate_inode_pages_range() to remove all folios from the page cache. This function then blocks indefinitely waiting to acquire the folio lock, while the readahead path is itself blocked in reclaim — creating a circular deadlock. The stack trace shows the deadlock path: folio_wait_bit_common → folio_lock → truncate_inode_pages_range → fuse_evict_inode → evict → shrink_dentry_list → prune_dcache_sb (Red Hat CVE).

Successful triggering of this vulnerability results in a kernel deadlock, causing a complete system hang (denial of service) on the affected host. The availability impact is severe, as the deadlocked kernel cannot recover without a reboot. There is no known confidentiality or integrity impact, and the vulnerability does not enable code execution or privilege escalation (Red Hat CVE).

The fix has been backported to multiple stable Linux kernel branches via patches at git.kernel.org (commits 4703bc0e, bd5603ea, cbbf3f1b, cf74785c, e0d6de83). Downstream distributions have released updated kernel packages: Ubuntu has issued security notices USN-8177-1, USN-8177-2, USN-8183-1, USN-8183-2, USN-8245-1, and USN-8257-1; Amazon Linux 2 has released ALAS2KERNEL-5.15-2026-098; and Red Hat has tracked the issue. Administrators should update to the latest patched kernel version provided by their Linux distribution as soon as possible (Red Hat CVE, Ubuntu USN-8177-1, Amazon ALAS).