cregit-Linux how code gets into the kernel

Release 4.15 kernel/umh.c

Directory: kernel
 * umh - the kernel usermode helper
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/binfmts.h>
#include <linux/syscalls.h>
#include <linux/unistd.h>
#include <linux/kmod.h>
#include <linux/slab.h>
#include <linux/completion.h>
#include <linux/cred.h>
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/workqueue.h>
#include <linux/security.h>
#include <linux/mount.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/resource.h>
#include <linux/notifier.h>
#include <linux/suspend.h>
#include <linux/rwsem.h>
#include <linux/ptrace.h>
#include <linux/async.h>
#include <linux/uaccess.h>

#include <trace/events/module.h>

#define CAP_BSET	(void *)1

#define CAP_PI		(void *)2

static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;

static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
static DEFINE_SPINLOCK(umh_sysctl_lock);
static DECLARE_RWSEM(umhelper_sem);

static void call_usermodehelper_freeinfo(struct subprocess_info *info) { if (info->cleanup) (*info->cleanup)(info); kfree(info); }


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static void umh_complete(struct subprocess_info *sub_info) { struct completion *comp = xchg(&sub_info->complete, NULL); /* * See call_usermodehelper_exec(). If xchg() returns NULL * we own sub_info, the UMH_KILLABLE caller has gone away * or the caller used UMH_NO_WAIT. */ if (comp) complete(comp); else call_usermodehelper_freeinfo(sub_info); }


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/* * This is the task which runs the usermode application */
static int call_usermodehelper_exec_async(void *data) { struct subprocess_info *sub_info = data; struct cred *new; int retval; spin_lock_irq(&current->sighand->siglock); flush_signal_handlers(current, 1); spin_unlock_irq(&current->sighand->siglock); /* * Our parent (unbound workqueue) runs with elevated scheduling * priority. Avoid propagating that into the userspace child. */ set_user_nice(current, 0); retval = -ENOMEM; new = prepare_kernel_cred(current); if (!new) goto out; spin_lock(&umh_sysctl_lock); new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset); new->cap_inheritable = cap_intersect(usermodehelper_inheritable, new->cap_inheritable); spin_unlock(&umh_sysctl_lock); if (sub_info->init) { retval = sub_info->init(sub_info, new); if (retval) { abort_creds(new); goto out; } } commit_creds(new); retval = do_execve(getname_kernel(sub_info->path), (const char __user *const __user *)sub_info->argv, (const char __user *const __user *)sub_info->envp); out: sub_info->retval = retval; /* * call_usermodehelper_exec_sync() will call umh_complete * if UHM_WAIT_PROC. */ if (!(sub_info->wait & UMH_WAIT_PROC)) umh_complete(sub_info); if (!retval) return 0; do_exit(0); }


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/* Handles UMH_WAIT_PROC. */
static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info) { pid_t pid; /* If SIGCLD is ignored sys_wait4 won't populate the status. */ kernel_sigaction(SIGCHLD, SIG_DFL); pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD); if (pid < 0) { sub_info->retval = pid; } else { int ret = -ECHILD; /* * Normally it is bogus to call wait4() from in-kernel because * wait4() wants to write the exit code to a userspace address. * But call_usermodehelper_exec_sync() always runs as kernel * thread (workqueue) and put_user() to a kernel address works * OK for kernel threads, due to their having an mm_segment_t * which spans the entire address space. * * Thus the __user pointer cast is valid here. */ sys_wait4(pid, (int __user *)&ret, 0, NULL); /* * If ret is 0, either call_usermodehelper_exec_async failed and * the real error code is already in sub_info->retval or * sub_info->retval is 0 anyway, so don't mess with it then. */ if (ret) sub_info->retval = ret; } /* Restore default kernel sig handler */ kernel_sigaction(SIGCHLD, SIG_IGN); umh_complete(sub_info); }


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/* * We need to create the usermodehelper kernel thread from a task that is affine * to an optimized set of CPUs (or nohz housekeeping ones) such that they * inherit a widest affinity irrespective of call_usermodehelper() callers with * possibly reduced affinity (eg: per-cpu workqueues). We don't want * usermodehelper targets to contend a busy CPU. * * Unbound workqueues provide such wide affinity and allow to block on * UMH_WAIT_PROC requests without blocking pending request (up to some limit). * * Besides, workqueues provide the privilege level that caller might not have * to perform the usermodehelper request. * */
static void call_usermodehelper_exec_work(struct work_struct *work) { struct subprocess_info *sub_info = container_of(work, struct subprocess_info, work); if (sub_info->wait & UMH_WAIT_PROC) { call_usermodehelper_exec_sync(sub_info); } else { pid_t pid; /* * Use CLONE_PARENT to reparent it to kthreadd; we do not * 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); if (pid < 0) { sub_info->retval = pid; umh_complete(sub_info); } } }


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/* * If set, call_usermodehelper_exec() will exit immediately returning -EBUSY * (used for preventing user land processes from being created after the user * land has been frozen during a system-wide hibernation or suspend operation). * Should always be manipulated under umhelper_sem acquired for write. */ static enum umh_disable_depth usermodehelper_disabled = UMH_DISABLED; /* Number of helpers running */ static atomic_t running_helpers = ATOMIC_INIT(0); /* * Wait queue head used by usermodehelper_disable() to wait for all running * helpers to finish. */ static DECLARE_WAIT_QUEUE_HEAD(running_helpers_waitq); /* * Used by usermodehelper_read_lock_wait() to wait for usermodehelper_disabled * to become 'false'. */ static DECLARE_WAIT_QUEUE_HEAD(usermodehelper_disabled_waitq); /* * Time to wait for running_helpers to become zero before the setting of * usermodehelper_disabled in usermodehelper_disable() fails */ #define RUNNING_HELPERS_TIMEOUT (5 * HZ)
int usermodehelper_read_trylock(void) { DEFINE_WAIT(wait); int ret = 0; down_read(&umhelper_sem); for (;;) { prepare_to_wait(&usermodehelper_disabled_waitq, &wait, TASK_INTERRUPTIBLE); if (!usermodehelper_disabled) break; if (usermodehelper_disabled == UMH_DISABLED) ret = -EAGAIN; up_read(&umhelper_sem); if (ret) break; schedule(); try_to_freeze(); down_read(&umhelper_sem); } finish_wait(&usermodehelper_disabled_waitq, &wait); return ret; }


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long usermodehelper_read_lock_wait(long timeout) { DEFINE_WAIT(wait); if (timeout < 0) return -EINVAL; down_read(&umhelper_sem); for (;;) { prepare_to_wait(&usermodehelper_disabled_waitq, &wait, TASK_UNINTERRUPTIBLE); if (!usermodehelper_disabled) break; up_read(&umhelper_sem); timeout = schedule_timeout(timeout); if (!timeout) break; down_read(&umhelper_sem); } finish_wait(&usermodehelper_disabled_waitq, &wait); return timeout; }


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void usermodehelper_read_unlock(void) { up_read(&umhelper_sem); }


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EXPORT_SYMBOL_GPL(usermodehelper_read_unlock); /** * __usermodehelper_set_disable_depth - Modify usermodehelper_disabled. * @depth: New value to assign to usermodehelper_disabled. * * Change the value of usermodehelper_disabled (under umhelper_sem locked for * writing) and wakeup tasks waiting for it to change. */
void __usermodehelper_set_disable_depth(enum umh_disable_depth depth) { down_write(&umhelper_sem); usermodehelper_disabled = depth; wake_up(&usermodehelper_disabled_waitq); up_write(&umhelper_sem); }


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/** * __usermodehelper_disable - Prevent new helpers from being started. * @depth: New value to assign to usermodehelper_disabled. * * Set usermodehelper_disabled to @depth and wait for running helpers to exit. */
int __usermodehelper_disable(enum umh_disable_depth depth) { long retval; if (!depth) return -EINVAL; down_write(&umhelper_sem); usermodehelper_disabled = depth; up_write(&umhelper_sem); /* * From now on call_usermodehelper_exec() won't start any new * helpers, so it is sufficient if running_helpers turns out to * be zero at one point (it may be increased later, but that * doesn't matter). */ retval = wait_event_timeout(running_helpers_waitq, atomic_read(&running_helpers) == 0, RUNNING_HELPERS_TIMEOUT); if (retval) return 0; __usermodehelper_set_disable_depth(UMH_ENABLED); return -EAGAIN; }


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static void helper_lock(void) { atomic_inc(&running_helpers); smp_mb__after_atomic(); }


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static void helper_unlock(void) { if (atomic_dec_and_test(&running_helpers)) wake_up(&running_helpers_waitq); }


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/** * call_usermodehelper_setup - prepare to call a usermode helper * @path: path to usermode executable * @argv: arg vector for process * @envp: environment for process * @gfp_mask: gfp mask for memory allocation * @cleanup: a cleanup function * @init: an init function * @data: arbitrary context sensitive data * * Returns either %NULL on allocation failure, or a subprocess_info * structure. This should be passed to call_usermodehelper_exec to * exec the process and free the structure. * * The init function is used to customize the helper process prior to * exec. A non-zero return code causes the process to error out, exit, * and return the failure to the calling process * * The cleanup function is just before ethe subprocess_info is about to * be freed. This can be used for freeing the argv and envp. The * Function must be runnable in either a process context or the * context in which call_usermodehelper_exec is called. */
struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv, char **envp, gfp_t gfp_mask, int (*init)(struct subprocess_info *info, struct cred *new), void (*cleanup)(struct subprocess_info *info), void *data) { struct subprocess_info *sub_info; sub_info = kzalloc(sizeof(struct subprocess_info), gfp_mask); if (!sub_info) goto out; INIT_WORK(&sub_info->work, call_usermodehelper_exec_work); #ifdef CONFIG_STATIC_USERMODEHELPER sub_info->path = CONFIG_STATIC_USERMODEHELPER_PATH; #else sub_info->path = path; #endif sub_info->argv = argv; sub_info->envp = envp; sub_info->cleanup = cleanup; sub_info->init = init; sub_info->data = data; out: return sub_info; }


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EXPORT_SYMBOL(call_usermodehelper_setup); /** * call_usermodehelper_exec - start a usermode application * @sub_info: information about the subprocessa * @wait: wait for the application to finish and return status. * when UMH_NO_WAIT don't wait at all, but you get no useful error back * when the program couldn't be exec'ed. This makes it safe to call * from interrupt context. * * Runs a user-space application. The application is started * asynchronously if wait is not set, and runs as a child of system workqueues. * (ie. it runs with full root capabilities and optimized affinity). */
int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait) { DECLARE_COMPLETION_ONSTACK(done); int retval = 0; if (!sub_info->path) { call_usermodehelper_freeinfo(sub_info); return -EINVAL; } helper_lock(); if (usermodehelper_disabled) { retval = -EBUSY; goto out; } /* * If there is no binary for us to call, then just return and get out of * here. This allows us to set STATIC_USERMODEHELPER_PATH to "" and * disable all call_usermodehelper() calls. */ if (strlen(sub_info->path) == 0) goto out; /* * Set the completion pointer only if there is a waiter. * This makes it possible to use umh_complete to free * the data structure in case of UMH_NO_WAIT. */ sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done; sub_info->wait = wait; queue_work(system_unbound_wq, &sub_info->work); if (wait == UMH_NO_WAIT) /* task has freed sub_info */ goto unlock; if (wait & UMH_KILLABLE) { retval = wait_for_completion_killable(&done); if (!retval) goto wait_done; /* umh_complete() will see NULL and free sub_info */ if (xchg(&sub_info->complete, NULL)) goto unlock; /* fallthrough, umh_complete() was already called */ } wait_for_completion(&done); wait_done: retval = sub_info->retval; out: call_usermodehelper_freeinfo(sub_info); unlock: helper_unlock(); return retval; }


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EXPORT_SYMBOL(call_usermodehelper_exec); /** * call_usermodehelper() - prepare and start a usermode application * @path: path to usermode executable * @argv: arg vector for process * @envp: environment for process * @wait: wait for the application to finish and return status. * when UMH_NO_WAIT don't wait at all, but you get no useful error back * when the program couldn't be exec'ed. This makes it safe to call * from interrupt context. * * This function is the equivalent to use call_usermodehelper_setup() and * call_usermodehelper_exec(). */
int call_usermodehelper(const char *path, char **argv, char **envp, int wait) { struct subprocess_info *info; gfp_t gfp_mask = (wait == UMH_NO_WAIT) ? GFP_ATOMIC : GFP_KERNEL; info = call_usermodehelper_setup(path, argv, envp, gfp_mask, NULL, NULL, NULL); if (info == NULL) return -ENOMEM; return call_usermodehelper_exec(info, wait); }


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static int proc_cap_handler(struct ctl_table *table, int write, void __user *buffer, size_t *lenp, loff_t *ppos) { struct ctl_table t; unsigned long cap_array[_KERNEL_CAPABILITY_U32S]; kernel_cap_t new_cap; int err, i; if (write && (!capable(CAP_SETPCAP) || !capable(CAP_SYS_MODULE))) return -EPERM; /* * convert from the global kernel_cap_t to the ulong array to print to * userspace if this is a read. */ spin_lock(&umh_sysctl_lock); for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++) { if (table->data == CAP_BSET) cap_array[i] = usermodehelper_bset.cap[i]; else if (table->data == CAP_PI) cap_array[i] = usermodehelper_inheritable.cap[i]; else BUG(); } spin_unlock(&umh_sysctl_lock); t = *table; = &cap_array; /* * actually read or write and array of ulongs from userspace. Remember * these are least significant 32 bits first */ err = proc_doulongvec_minmax(&t, write, buffer, lenp, ppos); if (err < 0) return err; /* * convert from the sysctl array of ulongs to the kernel_cap_t * internal representation */ for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++) new_cap.cap[i] = cap_array[i]; /* * Drop everything not in the new_cap (but don't add things) */ if (write) { spin_lock(&umh_sysctl_lock); if (table->data == CAP_BSET) usermodehelper_bset = cap_intersect(usermodehelper_bset, new_cap); if (table->data == CAP_PI) usermodehelper_inheritable = cap_intersect(usermodehelper_inheritable, new_cap); spin_unlock(&umh_sysctl_lock); } return 0; }


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Christophe Jaillet62.30%150.00%

struct ctl_table usermodehelper_table[] = { { .procname = "bset", .data = CAP_BSET, .maxlen = _KERNEL_CAPABILITY_U32S * sizeof(unsigned long), .mode = 0600, .proc_handler = proc_cap_handler, }, { .procname = "inheritable", .data = CAP_PI, .maxlen = _KERNEL_CAPABILITY_U32S * sizeof(unsigned long), .mode = 0600, .proc_handler = proc_cap_handler, }, { } };

Overall Contributors

Luis R. Rodriguez174599.66%150.00%
Christophe Jaillet60.34%150.00%
Directory: kernel
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