CVE-2026-98982namespace: don't let a pseudo dentry become the root of a mount

Official description

In the Linux kernel, the following vulnerability has been resolved: namespace: don't let a pseudo dentry become the root of a mount d_alloc_pseudo() hands out dentries for pipes, sockets and other files that are never anyone's child or parent. They carry DCACHE_NORCU and dentry_free() frees them right away because no lockless path walk can ever reach them. That holds as long as such a dentry isn't the root of a mount. But bind mounting /proc/self/fd/<fd> of such a file does exactly that. Most callers of alloc_file_pseudo() put their files on kernel-internal mounts and may_copy_tree() refuses those. bpf_token_create() doesn't. It places the token file on the bpffs mount the caller handed it and that mount is in the caller's mount namespace so the clone goes through: mount --bind /proc/self/fd/<token> <file> open_tree(tokfd, "", AT_EMPTY_PATH | OPEN_TREE_CLONE) + move_mount() __follow_mount_rcu() then loads ->mnt_root of that mount, reads d_seq and d_flags of the dentry and only then checks mount_lock. The dentry can be gone by then. The final mntput() of an unmounted parent unhooks a child that stayed attached to it under mount_lock alone. When the child's holder does the final put right after that the root is dput() and freed immediately while a walker that found the mount hashed still looks at it. Refuse to clone a mount with a DCACHE_NORCU dentry as its root. Nothing sensible can be done with a bind mount of a bpf token anyway.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/a256e31e0c54c4c37d2523aa52faabdcdd2ebc07
• https://git.kernel.org/stable/c/7e1c466011ba9e64d43075396badc03191bcee44
• https://git.kernel.org/stable/c/00287afa7cd989efdbf5822b7efac33cce7d489a
• https://git.kernel.org/stable/c/b138e16fb1e81376cea89f727dda7bfccd140de4