[PATCH RFC v3 20/26] fs: add umh argument to struct kernel_clone_args

Christian Brauner
2026-03-11T21:57:01Z

Add a umh field to struct kernel_clone_args. When set, copy_fs() copies
from pid 1's fs_struct instead of the kthread's fs_struct. This ensures
usermodehelper threads always get init's filesystem state regardless of
their parent's (kthreadd's) fs.

Usermodehelper threads are not allowed to create mount namespaces
(CLONE_NEWNS), share filesystem state (CLONE_FS), or be started from
a non-initial mount namespace. No usermodehelper currently does this so
we don't need to worry about this restriction.

Set .umh = 1 in user_mode_thread(). At this stage pid 1's fs points to
rootfs which is the same as kthreadd's fs, so this is functionally
equivalent.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
include/linux/sched/task.h | 1 +
kernel/fork.c | 25 +++++++++++++++++++++----
kernel/umh.c | 6 ++----
3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 41ed884cffc9..e0c1ca8c6a18 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -31,6 +31,7 @@ struct kernel_clone_args {
u32 io_thread:1;
u32 user_worker:1;
u32 no_files:1;
+ u32 umh:1;
unsigned long stack;
unsigned long stack_size;
unsigned long tls;
diff --git a/kernel/fork.c b/kernel/fork.c
index 154703cf7d3d..f62b4c370f74 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1590,11 +1590,27 @@ static int copy_mm(u64 clone_flags, struct task_struct *tsk)
return 0;
}

-static int copy_fs(u64 clone_flags, struct task_struct *tsk)
+static int copy_fs(u64 clone_flags, struct task_struct *tsk, bool umh)
{
- struct fs_struct *fs = current->fs;
+ struct fs_struct *fs;
+
+ /*
+ * Usermodehelper may use userspace_init_fs filesystem state but
+ * they don't get to create mount namespaces, share the
+ * filesystem state, or be started from a non-initial mount
+ * namespace.
+ */
+ if (umh) {
+ if (clone_flags & (CLONE_NEWNS | CLONE_FS))
+ return -EINVAL;
+ if (current->nsproxy->mnt_ns != &init_mnt_ns)
+ return -EINVAL;
+ fs = userspace_init_fs;
+ } else {
+ fs = current->fs;
+ VFS_WARN_ON_ONCE(current->fs != current->real_fs);
+ }

- VFS_WARN_ON_ONCE(current->fs != current->real_fs);
if (clone_flags & CLONE_FS) {
/* tsk->fs is already what we want */
read_seqlock_excl(&fs->seq);
@@ -2213,7 +2229,7 @@ __latent_entropy struct task_struct *copy_process(
retval = copy_files(clone_flags, p, args->no_files);
if (retval)
goto bad_fork_cleanup_semundo;
- retval = copy_fs(clone_flags, p);
+ retval = copy_fs(clone_flags, p, args->umh);
if (retval)
goto bad_fork_cleanup_files;
retval = copy_sighand(clone_flags, p);
@@ -2727,6 +2743,7 @@ pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags)
.exit_signal = (flags & CSIGNAL),
.fn = fn,
.fn_arg = arg,
+ .umh = 1,
};

return kernel_clone(&args);
diff --git a/kernel/umh.c b/kernel/umh.c
index cffda97d961c..d3f4b308b85d 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -71,10 +71,8 @@ static int call_usermodehelper_exec_async(void *data)
spin_unlock_irq(&current->sighand->siglock);

/*
- * Initial kernel threads share ther FS with init, in order to
- * get the init root directory. But we've now created a new
- * thread that is going to execve a user process and has its own
- * 'struct fs_struct'. Reset umask to the default.
+ * Usermodehelper threads get a copy of userspace init's
+ * fs_struct. Reset umask to the default.
*/
current->fs->umask = 0022;

--
2.47.3