Messages in this thread |  | | From | Christian Brauner <> | | Subject | [PATCH RFC v2 00/23] fs,kthread: start all kthreads in nullfs | | Date | Fri, 06 Mar 2026 00:30:03 +0100 |
| |
Summary:
* all kthreads are isolated in a separate SB_KERNMOUNT of nullfs. -> no lookup of anything else, no mounting on top of it, completely isolated. * init has a separate fs_struct from all kthreads * scoped_with_init_fs() allows a kthread to temporarily assume init's fs_struct for filesystem operations.
So this is a bit of a crazy series. When the kernel is started it roughly goes like this:
init_task ==> create pid 1 (systemd etc.) ==> pid 2 (kthreadd)
After this point all kthreads and PID 1 share the same filesystem state. That obviously already came up when we discussed pivot_root() as this allows pivot_root() to rewrite the fs_struct of all kthreads.
This rewriting is really weird and mostly done so kthread can use init's filesystem state when they would like to. But this really should be discouraged. The rewriting should also stop completely. I worked a bit to get rid of it in a more fundamental way. Is it crazy? Yes. Is it likely broken? Yes. Does it at least boot? Yes.
Instead of sharing fs_struct between kernel threads and pid 1, pid 1 get's a completely separate fs_struct. All kthreads continue sharing init_fs as before and pid 1's fs_struct is isolated from kthread's filesystem state. IOW, userspace init cannot affect kthreads filesystem state anymore and kthreads cannot affect userspace's filesystem state anymore - without explicit opt-in.
All kthreads are anchored in a kernel internal mount of nullfs that cannot be mounted on and that cannot be used to follow other mounts. It's a completely private mount that insulates kthreads.
This series makes performing mountains of filesystem work such as path lookup and file opening and so on from kthreads hard - painfully so. I think this is a benefit because it takes the idea of just offloading _security sensitive_ operations in init's filesystem state and running random binaries or opening and creating files to kthreads difficult behind the shed... And imho it should.
The only remaining kernel tasks that actually share init's filesystem state are usermodhelpers - as they execute random binaries in the root filesystem. Another concept we should really show the back of the shed.
This gives a lot stronger guarantees than what we have now. This also makes path lookup from kthreads fail by default. IOW, it won't be possible anymore to just lookup random stuff in init's filesytem state without explicitly opting in to that.
The places that need to perform lookup in init's filesystem state may use scoped_with_init_fs() which will temporarily override the caller's fs_struct with init's fs_struct.
We now also warn and notice when pid 1 simply stops sharing filesystem state with us, i.e., abandons it's userspace_init_fs.
On older kernels if PID 1 unshared its filesystem state with us the kernel simply used the stale fs_struct state implicitly pinning anything that PID 1 had last used. Even if PID 1 might've moved on to some completely different fs_struct state and might've even unmounted the old root.
This has hilarious consequences: Think continuing to dump coredump state into an implicitly pinned directory somewhere. Calling random binaries in the old rootfs via usermodehelpers.
Be aggressive about this: We simply reject operating on stale fs_struct state by reverting userspace_init_fs to nullfs. Every kworker that does lookups after this point will fail. Every usermodehelper call will fail. This is a lot stronger but I wouldn't know what it means for pid 1 to simply stop sharing its fs state with the kernel. Clearly it wanted to separate so cut all ties.
I've went through the kernel and looked at hopefully everything that does path lookup from kthreads (workqueues, ...).
TL;DR:
==== PID 1 (systemd) ====
root@localhost:~# stat --file-system /proc/1/root File: "/proc/1/root" ID: e3cb00dd533cd3d7 Namelen: 255 Type: ext2/ext3
root@localhost:~# cat /proc/1/mountinfo | wc -l 30
==== PID 2 (kthreadd) ====
root@localhost:~# stat --file-system /proc/2/root File: "/proc/2/root" ID: 200000000 Namelen: 255 Type: nullfs
root@localhost:~# cat /proc/2/mountinfo | wc -l 0
Signed-off-by: Christian Brauner <brauner@kernel.org> --- Changes in v2: - Remove LOOKUP_IN_INIT in favor of scoped_with_init_fs(). - Link to v1: https://patch.msgid.link/20260303-work-kthread-nullfs-v1-0-87e559b94375@kernel.org
--- Christian Brauner (23): fs: notice when init abandons fs sharing fs: add scoped_with_init_fs() rnbd: use scoped_with_init_fs() for block device open crypto: ccp: use scoped_with_init_fs() for SEV file access scsi: target: use scoped_with_init_fs() for ALUA metadata scsi: target: use scoped_with_init_fs() for APTPL metadata btrfs: use scoped_with_init_fs() for update_dev_time() coredump: use scoped_with_init_fs() for coredump path resolution fs: use scoped_with_init_fs() for kernel_read_file_from_path_initns() ksmbd: use scoped_with_init_fs() for share path resolution ksmbd: use scoped_with_init_fs() for filesystem info path lookup ksmbd: use scoped_with_init_fs() for VFS path operations initramfs: use scoped_with_init_fs() for rootfs unpacking af_unix: use scoped_with_init_fs() for coredump socket lookup fs: add real_fs to track task's actual fs_struct fs: make userspace_init_fs a dynamically-initialized pointer fs: stop sharing fs_struct between init_task and pid 1 fs: add umh argument to struct kernel_clone_args fs: add kthread_mntns() devtmpfs: create private mount namespace nullfs: make nullfs multi-instance fs: start all kthreads in nullfs fs: stop rewriting kthread fs structs
drivers/base/devtmpfs.c | 2 +- drivers/block/rnbd/rnbd-srv.c | 4 +- drivers/crypto/ccp/sev-dev.c | 12 ++--- drivers/target/target_core_alua.c | 6 ++- drivers/target/target_core_pr.c | 4 +- fs/btrfs/volumes.c | 11 ++++- fs/coredump.c | 11 ++--- fs/fs_struct.c | 96 ++++++++++++++++++++++++++++++++++++++- fs/kernel_read_file.c | 9 +--- fs/namespace.c | 40 ++++++++++++++-- fs/nullfs.c | 7 +-- fs/smb/server/mgmt/share_config.c | 4 +- fs/smb/server/smb2pdu.c | 4 +- fs/smb/server/vfs.c | 14 ++++-- include/linux/fs_struct.h | 34 ++++++++++++++ include/linux/init_task.h | 1 + include/linux/mount.h | 1 + include/linux/sched.h | 1 + include/linux/sched/task.h | 1 + init/init_task.c | 1 + init/initramfs.c | 12 +++-- init/main.c | 10 +++- kernel/fork.c | 41 +++++++++++------ net/unix/af_unix.c | 17 +++---- 24 files changed, 266 insertions(+), 77 deletions(-) --- base-commit: c107785c7e8dbabd1c18301a1c362544b5786282 change-id: 20260303-work-kthread-nullfs-875a837f4198
|  |