Commit message: fs: take i_mutex during prepare_binprm for set[ug]id executables

This prevents a race between chown() and execve(), where chowning a
setuid-user binary to root would momentarily make the binary setuid
root.

This patch was mostly written by Linus Torvalds.

Signed-off-by: Jann Horn <jann@thejh.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2015-3339
CWE: CWE-362
Vulnerability description: Race condition in the prepare_binprm function in fs/exec.c in the Linux kernel before 3.19.6 allows local users to gain privileges by executing a setuid program at a time instant when a chown to root is in progress, and the ownership is changed but the setuid bit is not yet stripped.

Project: linux
File: exec.c

COMPAT_SYSCALL_DEFINE3(execve, const char __user *, filename,
	const compat_uptr_t __user *, argv,
	const compat_uptr_t __user *, envp)
{
	return compat_do_execve(getname(filename), argv, envp);
}

Project: linux
File: exec.c

COMPAT_SYSCALL_DEFINE5(execveat, int, fd,
		       const char __user *, filename,
		       const compat_uptr_t __user *, argv,
		       const compat_uptr_t __user *, envp,
		       int,  flags)
{
	int lookup_flags = (flags & AT_EMPTY_PATH) ? LOOKUP_EMPTY : 0;

	return compat_do_execveat(fd,
				  getname_flags(filename, lookup_flags, NULL),
				  argv, envp, flags);
}

Project: linux
File: exec.c

SYSCALL_DEFINE1(uselib, const char __user *, library)
{
	struct linux_binfmt *fmt;
	struct file *file;
	struct filename *tmp = getname(library);
	int error = PTR_ERR(tmp);
	static const struct open_flags uselib_flags = {
		.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
		.acc_mode = MAY_READ | MAY_EXEC | MAY_OPEN,
		.intent = LOOKUP_OPEN,
		.lookup_flags = LOOKUP_FOLLOW,
	};

	if (IS_ERR(tmp))
		goto out;

	file = do_filp_open(AT_FDCWD, tmp, &uselib_flags);
	putname(tmp);
	error = PTR_ERR(file);
	if (IS_ERR(file))
		goto out;

	error = -EINVAL;
	if (!S_ISREG(file_inode(file)->i_mode))
		goto exit;

	error = -EACCES;
	if (file->f_path.mnt->mnt_flags & MNT_NOEXEC)
		goto exit;

	fsnotify_open(file);

	error = -ENOEXEC;

	read_lock(&binfmt_lock);
	list_for_each_entry(fmt, &formats, lh) {
		if (!fmt->load_shlib)
			continue;
		if (!try_module_get(fmt->module))
			continue;
		read_unlock(&binfmt_lock);
		error = fmt->load_shlib(file);
		read_lock(&binfmt_lock);
		put_binfmt(fmt);
		if (error != -ENOEXEC)
			break;
	}
	read_unlock(&binfmt_lock);
exit:
	fput(file);
out:
  	return error;
}

Project: linux
File: exec.c

static int __bprm_mm_init(struct linux_binprm *bprm)
{
	int err;
	struct vm_area_struct *vma = NULL;
	struct mm_struct *mm = bprm->mm;

	bprm->vma = vma = kmem_cache_zalloc(vm_area_cachep, GFP_KERNEL);
	if (!vma)
		return -ENOMEM;

	down_write(&mm->mmap_sem);
	vma->vm_mm = mm;

	/*
	 * Place the stack at the largest stack address the architecture
	 * supports. Later, we'll move this to an appropriate place. We don't
	 * use STACK_TOP because that can depend on attributes which aren't
	 * configured yet.
	 */
	BUILD_BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
	vma->vm_end = STACK_TOP_MAX;
	vma->vm_start = vma->vm_end - PAGE_SIZE;
	vma->vm_flags = VM_SOFTDIRTY | VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
	INIT_LIST_HEAD(&vma->anon_vma_chain);

	err = insert_vm_struct(mm, vma);
	if (err)
		goto err;

	mm->stack_vm = mm->total_vm = 1;
	arch_bprm_mm_init(mm, vma);
	up_write(&mm->mmap_sem);
	bprm->p = vma->vm_end - sizeof(void *);
	return 0;
err:
	up_write(&mm->mmap_sem);
	bprm->vma = NULL;
	kmem_cache_free(vm_area_cachep, vma);
	return err;
}

Project: linux
File: exec.c

void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
{
	task_lock(tsk);
	trace_task_rename(tsk, buf);
	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
	task_unlock(tsk);
	perf_event_comm(tsk, exec);
}

Project: linux
File: exec.c

static int bprm_mm_init(struct linux_binprm *bprm)
{
	int err;
	struct mm_struct *mm = NULL;

	bprm->mm = mm = mm_alloc();
	err = -ENOMEM;
	if (!mm)
		goto err;

	err = __bprm_mm_init(bprm);
	if (err)
		goto err;

	return 0;

err:
	if (mm) {
		bprm->mm = NULL;
		mmdrop(mm);
	}

	return err;
}

Project: linux
File: exec.c

static void check_unsafe_exec(struct linux_binprm *bprm)
{
	struct task_struct *p = current, *t;
	unsigned n_fs;

	if (p->ptrace) {
		if (p->ptrace & PT_PTRACE_CAP)
			bprm->unsafe |= LSM_UNSAFE_PTRACE_CAP;
		else
			bprm->unsafe |= LSM_UNSAFE_PTRACE;
	}

	/*
	 * This isn't strictly necessary, but it makes it harder for LSMs to
	 * mess up.
	 */
	if (task_no_new_privs(current))
		bprm->unsafe |= LSM_UNSAFE_NO_NEW_PRIVS;

	t = p;
	n_fs = 1;
	spin_lock(&p->fs->lock);
	rcu_read_lock();
	while_each_thread(p, t) {
		if (t->fs == p->fs)
			n_fs++;
	}
	rcu_read_unlock();

	if (p->fs->users > n_fs)
		bprm->unsafe |= LSM_UNSAFE_SHARE;
	else
		p->fs->in_exec = 1;
 	spin_unlock(&p->fs->lock);
 }

Project: linux
File: exec.c

static int compat_do_execve(struct filename *filename,
	const compat_uptr_t __user *__argv,
	const compat_uptr_t __user *__envp)
{
	struct user_arg_ptr argv = {
		.is_compat = true,
		.ptr.compat = __argv,
	};
	struct user_arg_ptr envp = {
		.is_compat = true,
		.ptr.compat = __envp,
	};
	return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
}

Project: linux
File: exec.c

static int compat_do_execveat(int fd, struct filename *filename,
			      const compat_uptr_t __user *__argv,
			      const compat_uptr_t __user *__envp,
			      int flags)
{
	struct user_arg_ptr argv = {
		.is_compat = true,
		.ptr.compat = __argv,
	};
	struct user_arg_ptr envp = {
		.is_compat = true,
		.ptr.compat = __envp,
	};
	return do_execveat_common(fd, filename, argv, envp, flags);
}

Project: linux
File: exec.c

static int de_thread(struct task_struct *tsk)
{
	struct signal_struct *sig = tsk->signal;
	struct sighand_struct *oldsighand = tsk->sighand;
	spinlock_t *lock = &oldsighand->siglock;

	if (thread_group_empty(tsk))
		goto no_thread_group;

	/*
	 * Kill all other threads in the thread group.
	 */
	spin_lock_irq(lock);
	if (signal_group_exit(sig)) {
		/*
		 * Another group action in progress, just
		 * return so that the signal is processed.
		 */
		spin_unlock_irq(lock);
		return -EAGAIN;
	}

	sig->group_exit_task = tsk;
	sig->notify_count = zap_other_threads(tsk);
	if (!thread_group_leader(tsk))
		sig->notify_count--;

	while (sig->notify_count) {
		__set_current_state(TASK_KILLABLE);
		spin_unlock_irq(lock);
		schedule();
		if (unlikely(__fatal_signal_pending(tsk)))
			goto killed;
		spin_lock_irq(lock);
	}
	spin_unlock_irq(lock);

	/*
	 * At this point all other threads have exited, all we have to
	 * do is to wait for the thread group leader to become inactive,
	 * and to assume its PID:
	 */
	if (!thread_group_leader(tsk)) {
		struct task_struct *leader = tsk->group_leader;

		for (;;) {
			threadgroup_change_begin(tsk);
			write_lock_irq(&tasklist_lock);
			/*
			 * Do this under tasklist_lock to ensure that
			 * exit_notify() can't miss ->group_exit_task
			 */
			sig->notify_count = -1;
			if (likely(leader->exit_state))
				break;
			__set_current_state(TASK_KILLABLE);
			write_unlock_irq(&tasklist_lock);
			threadgroup_change_end(tsk);
			schedule();
			if (unlikely(__fatal_signal_pending(tsk)))
				goto killed;
		}

		/*
		 * The only record we have of the real-time age of a
		 * process, regardless of execs it's done, is start_time.
		 * All the past CPU time is accumulated in signal_struct
		 * from sister threads now dead.  But in this non-leader
		 * exec, nothing survives from the original leader thread,
		 * whose birth marks the true age of this process now.
		 * When we take on its identity by switching to its PID, we
		 * also take its birthdate (always earlier than our own).
		 */
		tsk->start_time = leader->start_time;
		tsk->real_start_time = leader->real_start_time;

		BUG_ON(!same_thread_group(leader, tsk));
		BUG_ON(has_group_leader_pid(tsk));
		/*
		 * An exec() starts a new thread group with the
		 * TGID of the previous thread group. Rehash the
		 * two threads with a switched PID, and release
		 * the former thread group leader:
		 */

		/* Become a process group leader with the old leader's pid.
		 * The old leader becomes a thread of the this thread group.
		 * Note: The old leader also uses this pid until release_task
		 *       is called.  Odd but simple and correct.
		 */
		tsk->pid = leader->pid;
		change_pid(tsk, PIDTYPE_PID, task_pid(leader));
		transfer_pid(leader, tsk, PIDTYPE_PGID);
		transfer_pid(leader, tsk, PIDTYPE_SID);

		list_replace_rcu(&leader->tasks, &tsk->tasks);
		list_replace_init(&leader->sibling, &tsk->sibling);

		tsk->group_leader = tsk;
		leader->group_leader = tsk;

		tsk->exit_signal = SIGCHLD;
		leader->exit_signal = -1;

		BUG_ON(leader->exit_state != EXIT_ZOMBIE);
		leader->exit_state = EXIT_DEAD;

		/*
		 * We are going to release_task()->ptrace_unlink() silently,
		 * the tracer can sleep in do_wait(). EXIT_DEAD guarantees
		 * the tracer wont't block again waiting for this thread.
		 */
		if (unlikely(leader->ptrace))
			__wake_up_parent(leader, leader->parent);
		write_unlock_irq(&tasklist_lock);
		threadgroup_change_end(tsk);

		release_task(leader);
	}

	sig->group_exit_task = NULL;
	sig->notify_count = 0;

no_thread_group:
	/* we have changed execution domain */
	tsk->exit_signal = SIGCHLD;

	exit_itimers(sig);
	flush_itimer_signals();

	if (atomic_read(&oldsighand->count) != 1) {
		struct sighand_struct *newsighand;
		/*
		 * This ->sighand is shared with the CLONE_SIGHAND
		 * but not CLONE_THREAD task, switch to the new one.
		 */
		newsighand = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
		if (!newsighand)
			return -ENOMEM;

		atomic_set(&newsighand->count, 1);
		memcpy(newsighand->action, oldsighand->action,
		       sizeof(newsighand->action));

		write_lock_irq(&tasklist_lock);
		spin_lock(&oldsighand->siglock);
		rcu_assign_pointer(tsk->sighand, newsighand);
		spin_unlock(&oldsighand->siglock);
		write_unlock_irq(&tasklist_lock);

		__cleanup_sighand(oldsighand);
	}

	BUG_ON(!thread_group_leader(tsk));
	return 0;

killed:
	/* protects against exit_notify() and __exit_signal() */
	read_lock(&tasklist_lock);
	sig->group_exit_task = NULL;
	sig->notify_count = 0;
	read_unlock(&tasklist_lock);
	return -EAGAIN;
}

Project: linux
File: exec.c

int do_execve(struct filename *filename,
	const char __user *const __user *__argv,
	const char __user *const __user *__envp)
{
	struct user_arg_ptr argv = { .ptr.native = __argv };
	struct user_arg_ptr envp = { .ptr.native = __envp };
	return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
}

Project: linux
File: exec.c

int do_execveat(int fd, struct filename *filename,
		const char __user *const __user *__argv,
		const char __user *const __user *__envp,
		int flags)
{
	struct user_arg_ptr argv = { .ptr.native = __argv };
	struct user_arg_ptr envp = { .ptr.native = __envp };

	return do_execveat_common(fd, filename, argv, envp, flags);
}

Project: linux
File: exec.c

static int do_execveat_common(int fd, struct filename *filename,
			      struct user_arg_ptr argv,
			      struct user_arg_ptr envp,
			      int flags)
{
	char *pathbuf = NULL;
	struct linux_binprm *bprm;
	struct file *file;
	struct files_struct *displaced;
	int retval;

	if (IS_ERR(filename))
		return PTR_ERR(filename);

	/*
	 * We move the actual failure in case of RLIMIT_NPROC excess from
	 * set*uid() to execve() because too many poorly written programs
	 * don't check setuid() return code.  Here we additionally recheck
	 * whether NPROC limit is still exceeded.
	 */
	if ((current->flags & PF_NPROC_EXCEEDED) &&
	    atomic_read(&current_user()->processes) > rlimit(RLIMIT_NPROC)) {
		retval = -EAGAIN;
		goto out_ret;
	}

	/* We're below the limit (still or again), so we don't want to make
	 * further execve() calls fail. */
	current->flags &= ~PF_NPROC_EXCEEDED;

	retval = unshare_files(&displaced);
	if (retval)
		goto out_ret;

	retval = -ENOMEM;
	bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
	if (!bprm)
		goto out_files;

	retval = prepare_bprm_creds(bprm);
	if (retval)
		goto out_free;

	check_unsafe_exec(bprm);
	current->in_execve = 1;

	file = do_open_execat(fd, filename, flags);
	retval = PTR_ERR(file);
	if (IS_ERR(file))
		goto out_unmark;

	sched_exec();

	bprm->file = file;
	if (fd == AT_FDCWD || filename->name[0] == '/') {
		bprm->filename = filename->name;
	} else {
		if (filename->name[0] == '\0')
			pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d", fd);
		else
			pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d/%s",
					    fd, filename->name);
		if (!pathbuf) {
			retval = -ENOMEM;
			goto out_unmark;
		}
		/*
		 * Record that a name derived from an O_CLOEXEC fd will be
		 * inaccessible after exec. Relies on having exclusive access to
		 * current->files (due to unshare_files above).
		 */
		if (close_on_exec(fd, rcu_dereference_raw(current->files->fdt)))
			bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;
		bprm->filename = pathbuf;
	}
	bprm->interp = bprm->filename;

	retval = bprm_mm_init(bprm);
	if (retval)
		goto out_unmark;

	bprm->argc = count(argv, MAX_ARG_STRINGS);
	if ((retval = bprm->argc) < 0)
		goto out;

	bprm->envc = count(envp, MAX_ARG_STRINGS);
	if ((retval = bprm->envc) < 0)
		goto out;

	retval = prepare_binprm(bprm);
	if (retval < 0)
		goto out;

	retval = copy_strings_kernel(1, &bprm->filename, bprm);
	if (retval < 0)
		goto out;

	bprm->exec = bprm->p;
	retval = copy_strings(bprm->envc, envp, bprm);
	if (retval < 0)
		goto out;

	retval = copy_strings(bprm->argc, argv, bprm);
	if (retval < 0)
		goto out;

	retval = exec_binprm(bprm);
	if (retval < 0)
		goto out;

	/* execve succeeded */
	current->fs->in_exec = 0;
	current->in_execve = 0;
	acct_update_integrals(current);
	task_numa_free(current);
	free_bprm(bprm);
	kfree(pathbuf);
	putname(filename);
	if (displaced)
		put_files_struct(displaced);
	return retval;

out:
	if (bprm->mm) {
		acct_arg_size(bprm, 0);
		mmput(bprm->mm);
	}

out_unmark:
	current->fs->in_exec = 0;
	current->in_execve = 0;

out_free:
	free_bprm(bprm);
	kfree(pathbuf);

out_files:
	if (displaced)
		reset_files_struct(displaced);
out_ret:
	putname(filename);
	return retval;
}

Project: linux
File: exec.c

static int exec_binprm(struct linux_binprm *bprm)
{
	pid_t old_pid, old_vpid;
	int ret;

	/* Need to fetch pid before load_binary changes it */
	old_pid = current->pid;
	rcu_read_lock();
	old_vpid = task_pid_nr_ns(current, task_active_pid_ns(current->parent));
	rcu_read_unlock();

	ret = search_binary_handler(bprm);
	if (ret >= 0) {
		audit_bprm(bprm);
		trace_sched_process_exec(current, old_pid, bprm);
		ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
		proc_exec_connector(current);
	}

	return ret;
}

Project: linux
File: exec.c

static int exec_mmap(struct mm_struct *mm)
{
	struct task_struct *tsk;
	struct mm_struct *old_mm, *active_mm;

	/* Notify parent that we're no longer interested in the old VM */
	tsk = current;
	old_mm = current->mm;
	mm_release(tsk, old_mm);

	if (old_mm) {
		sync_mm_rss(old_mm);
		/*
		 * Make sure that if there is a core dump in progress
		 * for the old mm, we get out and die instead of going
		 * through with the exec.  We must hold mmap_sem around
		 * checking core_state and changing tsk->mm.
		 */
		down_read(&old_mm->mmap_sem);
		if (unlikely(old_mm->core_state)) {
			up_read(&old_mm->mmap_sem);
			return -EINTR;
		}
	}
	task_lock(tsk);
	active_mm = tsk->active_mm;
	tsk->mm = mm;
	tsk->active_mm = mm;
	activate_mm(active_mm, mm);
	tsk->mm->vmacache_seqnum = 0;
	vmacache_flush(tsk);
	task_unlock(tsk);
	if (old_mm) {
		up_read(&old_mm->mmap_sem);
		BUG_ON(active_mm != old_mm);
		setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm);
		mm_update_next_owner(old_mm);
		mmput(old_mm);
		return 0;
	}
	mmdrop(active_mm);
	return 0;
}

Project: linux
File: exec.c

int flush_old_exec(struct linux_binprm * bprm)
{
	int retval;

	/*
	 * Make sure we have a private signal table and that
	 * we are unassociated from the previous thread group.
	 */
	retval = de_thread(current);
	if (retval)
		goto out;

	/*
	 * Must be called _before_ exec_mmap() as bprm->mm is
	 * not visibile until then. This also enables the update
	 * to be lockless.
	 */
	set_mm_exe_file(bprm->mm, bprm->file);

	/*
	 * Release all of the old mmap stuff
	 */
	acct_arg_size(bprm, 0);
	retval = exec_mmap(bprm->mm);
	if (retval)
		goto out;

	bprm->mm = NULL;		/* We're using it now */

	set_fs(USER_DS);
	current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
					PF_NOFREEZE | PF_NO_SETAFFINITY);
	flush_thread();
	current->personality &= ~bprm->per_clear;

	return 0;

out:
	return retval;
}

Project: linux
File: exec.c

static void free_bprm(struct linux_binprm *bprm)
{
	free_arg_pages(bprm);
	if (bprm->cred) {
		mutex_unlock(&current->signal->cred_guard_mutex);
		abort_creds(bprm->cred);
	}
	if (bprm->file) {
		allow_write_access(bprm->file);
		fput(bprm->file);
	}
	/* If a binfmt changed the interp, free it. */
	if (bprm->interp != bprm->filename)
		kfree(bprm->interp);
	kfree(bprm);
}

Project: linux
File: exec.c

struct file *open_exec(const char *name)
{
	struct filename *filename = getname_kernel(name);
	struct file *f = ERR_CAST(filename);

	if (!IS_ERR(filename)) {
		f = do_open_execat(AT_FDCWD, filename, 0);
		putname(filename);
	}
	return f;
}

Project: linux
File: exec.c

ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
{
	ssize_t res = vfs_read(file, (void __user *)addr, len, &pos);
	if (res > 0)
		flush_icache_range(addr, addr + len);
	return res;
}

Project: linux
File: exec.c

int search_binary_handler(struct linux_binprm *bprm)
{
	bool need_retry = IS_ENABLED(CONFIG_MODULES);
	struct linux_binfmt *fmt;
	int retval;

	/* This allows 4 levels of binfmt rewrites before failing hard. */
	if (bprm->recursion_depth > 5)
		return -ELOOP;

	retval = security_bprm_check(bprm);
	if (retval)
		return retval;

	retval = -ENOENT;
 retry:
	read_lock(&binfmt_lock);
	list_for_each_entry(fmt, &formats, lh) {
		if (!try_module_get(fmt->module))
			continue;
		read_unlock(&binfmt_lock);
		bprm->recursion_depth++;
		retval = fmt->load_binary(bprm);
		read_lock(&binfmt_lock);
		put_binfmt(fmt);
		bprm->recursion_depth--;
		if (retval < 0 && !bprm->mm) {
			/* we got to flush_old_exec() and failed after it */
			read_unlock(&binfmt_lock);
			force_sigsegv(SIGSEGV, current);
			return retval;
		}
		if (retval != -ENOEXEC || !bprm->file) {
			read_unlock(&binfmt_lock);
			return retval;
		}
	}
	read_unlock(&binfmt_lock);

	if (need_retry) {
		if (printable(bprm->buf[0]) && printable(bprm->buf[1]) &&
		    printable(bprm->buf[2]) && printable(bprm->buf[3]))
			return retval;
		if (request_module("binfmt-%04x", *(ushort *)(bprm->buf + 2)) < 0)
			return retval;
		need_retry = false;
		goto retry;
	}

	return retval;
}

Project: linux
File: exec.c

void set_dumpable(struct mm_struct *mm, int value)
{
	unsigned long old, new;

	if (WARN_ON((unsigned)value > SUID_DUMP_ROOT))
		return;

	do {
		old = ACCESS_ONCE(mm->flags);
		new = (old & ~MMF_DUMPABLE_MASK) | value;
	} while (cmpxchg(&mm->flags, old, new) != old);
}

Project: linux
File: exec.c

int setup_arg_pages(struct linux_binprm *bprm,
		    unsigned long stack_top,
		    int executable_stack)
{
	unsigned long ret;
	unsigned long stack_shift;
	struct mm_struct *mm = current->mm;
	struct vm_area_struct *vma = bprm->vma;
	struct vm_area_struct *prev = NULL;
	unsigned long vm_flags;
	unsigned long stack_base;
	unsigned long stack_size;
	unsigned long stack_expand;
	unsigned long rlim_stack;

#ifdef CONFIG_STACK_GROWSUP
	/* Limit stack size */
	stack_base = rlimit_max(RLIMIT_STACK);
	if (stack_base > STACK_SIZE_MAX)
		stack_base = STACK_SIZE_MAX;

	/* Make sure we didn't let the argument array grow too large. */
	if (vma->vm_end - vma->vm_start > stack_base)
		return -ENOMEM;

	stack_base = PAGE_ALIGN(stack_top - stack_base);

	stack_shift = vma->vm_start - stack_base;
	mm->arg_start = bprm->p - stack_shift;
	bprm->p = vma->vm_end - stack_shift;
#else
	stack_top = arch_align_stack(stack_top);
	stack_top = PAGE_ALIGN(stack_top);

	if (unlikely(stack_top < mmap_min_addr) ||
	    unlikely(vma->vm_end - vma->vm_start >= stack_top - mmap_min_addr))
		return -ENOMEM;

	stack_shift = vma->vm_end - stack_top;

	bprm->p -= stack_shift;
	mm->arg_start = bprm->p;
#endif

	if (bprm->loader)
		bprm->loader -= stack_shift;
	bprm->exec -= stack_shift;

	down_write(&mm->mmap_sem);
	vm_flags = VM_STACK_FLAGS;

	/*
	 * Adjust stack execute permissions; explicitly enable for
	 * EXSTACK_ENABLE_X, disable for EXSTACK_DISABLE_X and leave alone
	 * (arch default) otherwise.
	 */
	if (unlikely(executable_stack == EXSTACK_ENABLE_X))
		vm_flags |= VM_EXEC;
	else if (executable_stack == EXSTACK_DISABLE_X)
		vm_flags &= ~VM_EXEC;
	vm_flags |= mm->def_flags;
	vm_flags |= VM_STACK_INCOMPLETE_SETUP;

	ret = mprotect_fixup(vma, &prev, vma->vm_start, vma->vm_end,
			vm_flags);
	if (ret)
		goto out_unlock;
	BUG_ON(prev != vma);

	/* Move stack pages down in memory. */
	if (stack_shift) {
		ret = shift_arg_pages(vma, stack_shift);
		if (ret)
			goto out_unlock;
	}

	/* mprotect_fixup is overkill to remove the temporary stack flags */
	vma->vm_flags &= ~VM_STACK_INCOMPLETE_SETUP;

	stack_expand = 131072UL; /* randomly 32*4k (or 2*64k) pages */
	stack_size = vma->vm_end - vma->vm_start;
	/*
	 * Align this down to a page boundary as expand_stack
	 * will align it up.
	 */
	rlim_stack = rlimit(RLIMIT_STACK) & PAGE_MASK;
#ifdef CONFIG_STACK_GROWSUP
	if (stack_size + stack_expand > rlim_stack)
		stack_base = vma->vm_start + rlim_stack;
	else
		stack_base = vma->vm_end + stack_expand;
#else
	if (stack_size + stack_expand > rlim_stack)
		stack_base = vma->vm_end - rlim_stack;
	else
		stack_base = vma->vm_start - stack_expand;
#endif
	current->mm->start_stack = bprm->p;
	ret = expand_stack(vma, stack_base);
	if (ret)
		ret = -EFAULT;

out_unlock:
	up_write(&mm->mmap_sem);
	return ret;
}

Project: linux
File: exec.c

void setup_new_exec(struct linux_binprm * bprm)
{
	arch_pick_mmap_layout(current->mm);

	/* This is the point of no return */
	current->sas_ss_sp = current->sas_ss_size = 0;

	if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid()))
		set_dumpable(current->mm, SUID_DUMP_USER);
	else
		set_dumpable(current->mm, suid_dumpable);

	perf_event_exec();
	__set_task_comm(current, kbasename(bprm->filename), true);

	/* Set the new mm task size. We have to do that late because it may
	 * depend on TIF_32BIT which is only updated in flush_thread() on
	 * some architectures like powerpc
	 */
	current->mm->task_size = TASK_SIZE;

	/* install the new credentials */
	if (!uid_eq(bprm->cred->uid, current_euid()) ||
	    !gid_eq(bprm->cred->gid, current_egid())) {
		current->pdeath_signal = 0;
	} else {
		would_dump(bprm, bprm->file);
		if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP)
			set_dumpable(current->mm, suid_dumpable);
	}

	/* An exec changes our domain. We are no longer part of the thread
	   group */
	current->self_exec_id++;
	flush_signal_handlers(current, 0);
	do_close_on_exec(current->files);
}