In the Linux kernel, the following vulnerability has been resolved: readdir: take no inode lock on an immutable directory iterate_dir() takes the… (CVE-2026-99084)

The Linux kernel vulnerability CVE-2026-99084 concerns the readdir function's inode locking behavior on immutable directories. Specifically, iterate_dir() took a shared i_rwsem lock on directories that never have entries and are never removed, such as nullfs directories used as roots of empty mount namespaces. This lock ordering caused readers and writers to block each other unnecessarily, potentially stalling all users sharing the inode. The fix introduces the FOP_IMMUTABLE flag for directories that never change, allowing iterate_dir() to skip taking the inode lock, thus eliminating the blocking issue. The flag is set on nullfs directories, and the patch ensures that other directory operations do not interfere with this behavior.

The vulnerability could cause system stalls or delays in mount namespaces sharing the same immutable directory inode, affecting kernel threads and users relying on these namespaces. While it does not directly lead to privilege escalation or data corruption, the blocking behavior can degrade system responsiveness and impact workloads relying on mount namespaces. No known exploits in the wild have been reported.

A fix has been implemented in the Linux kernel to address this issue by adding the FOP_IMMUTABLE flag and modifying iterate_dir() to skip inode locking on immutable directories. Users should apply the official kernel update containing this fix once available. Patch status is not explicitly confirmed in the provided data; therefore, check the vendor or kernel advisory for the current remediation guidance and apply updates accordingly.