namespace: don't let a pseudo dentry become the root of a mount

Overview

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

• Linux >= 35f96de04127d332a5c5e8a155d31f452f88c76d < a256e31e0c54c4c37d2523aa52faabdcdd2ebc07
• Linux >= 35f96de04127d332a5c5e8a155d31f452f88c76d < 7e1c466011ba9e64d43075396badc03191bcee44
• Linux >= 35f96de04127d332a5c5e8a155d31f452f88c76d < 00287afa7cd989efdbf5822b7efac33cce7d489a
• Linux >= 35f96de04127d332a5c5e8a155d31f452f88c76d < b138e16fb1e81376cea89f727dda7bfccd140de4
• Linux 6.9

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.