arch/um/kernel/skas/mmu.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2015 Thomas Meyer (thomas@m3y3r.de)
 * Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
 */

#include <linux/mm.h>
#include <linux/sched/signal.h>
#include <linux/slab.h>

#include <shared/irq_kern.h>
#include <asm/pgalloc.h>
#include <asm/sections.h>
#include <asm/mmu_context.h>
#include <as-layout.h>
#include <os.h>
#include <skas.h>
#include <stub-data.h>

/* Ensure the stub_data struct covers the allocated area */
static_assert(sizeof(struct stub_data) == STUB_DATA_PAGES * UM_KERN_PAGE_SIZE);

static spinlock_t mm_list_lock;
static struct list_head mm_list;

struct mutex *__get_turnstile(struct mm_id *mm_id)
{
	struct mm_context *ctx = container_of(mm_id, struct mm_context, id);

	return &ctx->turnstile;
}

void enter_turnstile(struct mm_id *mm_id)
{
	mutex_lock(__get_turnstile(mm_id));
}

void exit_turnstile(struct mm_id *mm_id)
{
	mutex_unlock(__get_turnstile(mm_id));
}

int init_new_context(struct task_struct *task, struct mm_struct *mm)
{
	struct mm_id *new_id = &mm->context.id;
	unsigned long stack = 0;
	int ret = -ENOMEM;

	mutex_init(&mm->context.turnstile);
	spin_lock_init(&mm->context.sync_tlb_lock);

	stack = __get_free_pages(GFP_KERNEL | __GFP_ZERO, ilog2(STUB_DATA_PAGES));
	if (stack == 0)
		goto out;

	new_id->stack = stack;
	new_id->syscall_data_len = 0;
	new_id->syscall_fd_num = 0;

	scoped_guard(spinlock_irqsave, &mm_list_lock) {
		/* Insert into list, used for lookups when the child dies */
		list_add(&mm->context.list, &mm_list);
	}

	ret = start_userspace(new_id);
	if (ret < 0)
		goto out_free;

	/* Ensure the new MM is clean and nothing unwanted is mapped */
	unmap(new_id, 0, STUB_START);

	return 0;

 out_free:
	free_pages(new_id->stack, ilog2(STUB_DATA_PAGES));
 out:
	return ret;
}

void destroy_context(struct mm_struct *mm)
{
	struct mm_context *mmu = &mm->context;

	/*
	 * If init_new_context wasn't called, this will be
	 * zero, resulting in a kill(0), which will result in the
	 * whole UML suddenly dying.  Also, cover negative and
	 * 1 cases, since they shouldn't happen either.
	 *
	 * Negative cases happen if the child died unexpectedly.
	 */
	if (mmu->id.pid >= 0 && mmu->id.pid < 2) {
		printk(KERN_ERR "corrupt mm_context - pid = %d\n",
		       mmu->id.pid);
		return;
	}

	scoped_guard(spinlock_irqsave, &mm_list_lock)
		list_del(&mm->context.list);

	if (mmu->id.pid > 0) {
		os_kill_ptraced_process(mmu->id.pid, 1);
		mmu->id.pid = -1;
	}

	if (using_seccomp && mmu->id.sock)
		os_close_file(mmu->id.sock);

	free_pages(mmu->id.stack, ilog2(STUB_DATA_PAGES));
}

static irqreturn_t mm_sigchld_irq(int irq, void* dev)
{
	struct mm_context *mm_context;
	pid_t pid;

	guard(spinlock)(&mm_list_lock);

	while ((pid = os_reap_child()) > 0) {
		/*
		* A child died, check if we have an MM with the PID. This is
		* only relevant in SECCOMP mode (as ptrace will fail anyway).
		*
		* See wait_stub_done_seccomp for more details.
		*/
		list_for_each_entry(mm_context, &mm_list, list) {
			if (mm_context->id.pid == pid) {
				struct stub_data *stub_data;
				printk("Unexpectedly lost MM child! Affected tasks will segfault.");

				/* Marks the MM as dead */
				mm_context->id.pid = -1;

				stub_data = (void *)mm_context->id.stack;
				stub_data->futex = FUTEX_IN_KERN;
#if IS_ENABLED(CONFIG_SMP)
				os_futex_wake(&stub_data->futex);
#endif

				/*
				 * NOTE: Currently executing syscalls by
				 * affected tasks may finish normally.
				 */
				break;
			}
		}
	}

	return IRQ_HANDLED;
}

static int __init init_child_tracking(void)
{
	int err;

	spin_lock_init(&mm_list_lock);
	INIT_LIST_HEAD(&mm_list);

	err = request_irq(SIGCHLD_IRQ, mm_sigchld_irq, 0, "SIGCHLD", NULL);
	if (err < 0)
		panic("Failed to register SIGCHLD IRQ: %d", err);

	return 0;
}
early_initcall(init_child_tracking)