[PATCH 5.17 038/772] kthread: Dont allocate kthread_struct for init and umh

Greg Kroah-Hartman
2022-06-07T20:28:37Z

From: Eric W. Biederman <ebiederm@xmission.com>

commit 343f4c49f2438d8920f1f76fa823ee59b91f02e4 upstream.

If kthread_is_per_cpu runs concurrently with free_kthread_struct the
kthread_struct that was just freed may be read from.

This bug was introduced by commit 40966e316f86 ("kthread: Ensure
struct kthread is present for all kthreads"). When kthread_struct
started to be allocated for all tasks that have PF_KTHREAD set. This
in turn required the kthread_struct to be freed in kernel_execve and
violated the assumption that kthread_struct will have the same
lifetime as the task.

Looking a bit deeper this only applies to callers of kernel_execve
which is just the init process and the user mode helper processes.
These processes really don't want to be kernel threads but are for
historical reasons. Mostly that copy_thread does not know how to take
a kernel mode function to the process with for processes without
PF_KTHREAD or PF_IO_WORKER set.

Solve this by not allocating kthread_struct for the init process and
the user mode helper processes.

This is done by adding a kthread member to struct kernel_clone_args.
Setting kthread in fork_idle and kernel_thread. Adding
user_mode_thread that works like kernel_thread except it does not set
kthread. In fork only allocating the kthread_struct if .kthread is set.

I have looked at kernel/kthread.c and since commit 40966e316f86
("kthread: Ensure struct kthread is present for all kthreads") there
have been no assumptions added that to_kthread or __to_kthread will
not return NULL.

There are a few callers of to_kthread or __to_kthread that assume a
non-NULL struct kthread pointer will be returned. These functions are
kthread_data(), kthread_parmme(), kthread_exit(), kthread(),
kthread_park(), kthread_unpark(), kthread_stop(). All of those functions
can reasonably expected to be called when it is know that a task is a
kthread so that assumption seems reasonable.

Cc: stable@vger.kernel.org
Fixes: 40966e316f86 ("kthread: Ensure struct kthread is present for all kthreads")
Reported-by: Максим Кутявин <maximkabox13@gmail.com>
Link: https://lkml.kernel.org/r/20220506141512.516114-1-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/exec.c | 6 ++++--
include/linux/sched/task.h | 2 ++
init/main.c | 2 +-
kernel/fork.c | 22 ++++++++++++++++++++--
kernel/umh.c | 6 +++---
5 files changed, 30 insertions(+), 8 deletions(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1315,8 +1315,6 @@ int begin_new_exec(struct linux_binprm *
*/
force_uaccess_begin();

- if (me->flags & PF_KTHREAD)
- free_kthread_struct(me);
me->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
PF_NOFREEZE | PF_NO_SETAFFINITY);
flush_thread();
@@ -1962,6 +1960,10 @@ int kernel_execve(const char *kernel_fil
int fd = AT_FDCWD;
int retval;

+ if (WARN_ON_ONCE((current->flags & PF_KTHREAD) &&
+ (current->worker_private)))
+ return -EINVAL;
+
filename = getname_kernel(kernel_filename);
if (IS_ERR(filename))
return PTR_ERR(filename);
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -32,6 +32,7 @@ struct kernel_clone_args {
size_t set_tid_size;
int cgroup;
int io_thread;
+ int kthread;
struct cgroup *cgrp;
struct css_set *cset;
};
@@ -89,6 +90,7 @@ struct task_struct *create_io_thread(int
struct task_struct *fork_idle(int);
struct mm_struct *copy_init_mm(void);
extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
+extern pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags);
extern long kernel_wait4(pid_t, int __user *, int, struct rusage *);
int kernel_wait(pid_t pid, int *stat);

--- a/init/main.c
+++ b/init/main.c
@@ -688,7 +688,7 @@ noinline void __ref rest_init(void)
* the init task will end up wanting to create kthreads, which, if
* we schedule it before we create kthreadd, will OOPS.
*/
- pid = kernel_thread(kernel_init, NULL, CLONE_FS);
+ pid = user_mode_thread(kernel_init, NULL, CLONE_FS);
/*
* Pin init on the boot CPU. Task migration is not properly working
* until sched_init_smp() has been run. It will set the allowed
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2087,7 +2087,7 @@ static __latent_entropy struct task_stru
p->io_context = NULL;
audit_set_context(p, NULL);
cgroup_fork(p);
- if (p->flags & PF_KTHREAD) {
+ if (args->kthread) {
if (!set_kthread_struct(p))
goto bad_fork_cleanup_delayacct;
}
@@ -2474,7 +2474,8 @@ struct task_struct * __init fork_idle(in
{
struct task_struct *task;
struct kernel_clone_args args = {
- .flags = CLONE_VM,
+ .flags = CLONE_VM,
+ .kthread = 1,
};

task = copy_process(&init_struct_pid, 0, cpu_to_node(cpu), &args);
@@ -2608,6 +2609,23 @@ pid_t kernel_thread(int (*fn)(void *), v
{
struct kernel_clone_args args = {
.flags = ((lower_32_bits(flags) | CLONE_VM |
+ CLONE_UNTRACED) & ~CSIGNAL),
+ .exit_signal = (lower_32_bits(flags) & CSIGNAL),
+ .stack = (unsigned long)fn,
+ .stack_size = (unsigned long)arg,
+ .kthread = 1,
+ };
+
+ return kernel_clone(&args);
+}
+
+/*
+ * Create a user mode thread.
+ */
+pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags)
+{
+ struct kernel_clone_args args = {
+ .flags = ((lower_32_bits(flags) | CLONE_VM |
CLONE_UNTRACED) & ~CSIGNAL),
.exit_signal = (lower_32_bits(flags) & CSIGNAL),
.stack = (unsigned long)fn,
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -132,7 +132,7 @@ static void call_usermodehelper_exec_syn

/* If SIGCLD is ignored do_wait won't populate the status. */
kernel_sigaction(SIGCHLD, SIG_DFL);
- pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);
+ pid = user_mode_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);
if (pid < 0)
sub_info->retval = pid;
else
@@ -171,8 +171,8 @@ static void call_usermodehelper_exec_wor
* want to pollute current->children, and we need a parent
* that always ignores SIGCHLD to ensure auto-reaping.
*/
- pid = kernel_thread(call_usermodehelper_exec_async, sub_info,
- CLONE_PARENT | SIGCHLD);
+ pid = user_mode_thread(call_usermodehelper_exec_async, sub_info,
+ CLONE_PARENT | SIGCHLD);
if (pid < 0) {
sub_info->retval = pid;
umh_complete(sub_info);