Re: [PATCH v3 4/8] iommufd: Add iommufd fault object

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Jan 22, 2024 at 03:38:59PM +0800, Lu Baolu wrote:
> --- /dev/null
> +++ b/drivers/iommu/iommufd/fault.c
> @@ -0,0 +1,255 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright (C) 2024 Intel Corporation
> + */
> +#define pr_fmt(fmt) "iommufd: " fmt
> +
> +#include <linux/file.h>
> +#include <linux/fs.h>
> +#include <linux/module.h>
> +#include <linux/mutex.h>
> +#include <linux/iommufd.h>
> +#include <linux/poll.h>
> +#include <linux/anon_inodes.h>
> +#include <uapi/linux/iommufd.h>
> +
> +#include "iommufd_private.h"
> +
> +static int device_add_fault(struct iopf_group *group)
> +{
> +	struct iommufd_device *idev = group->cookie->private;
> +	void *curr;
> +
> +	curr = xa_cmpxchg(&idev->faults, group->last_fault.fault.prm.grpid,
> +			  NULL, group, GFP_KERNEL);
> +
> +	return curr ? xa_err(curr) : 0;
> +}
> +
> +static void device_remove_fault(struct iopf_group *group)
> +{
> +	struct iommufd_device *idev = group->cookie->private;
> +
> +	xa_store(&idev->faults, group->last_fault.fault.prm.grpid,
> +		 NULL, GFP_KERNEL);

xa_erase ?

Is grpid OK to use this way? Doesn't it come from the originating
device?

> +void iommufd_fault_destroy(struct iommufd_object *obj)
> +{
> +	struct iommufd_fault *fault = container_of(obj, struct iommufd_fault, obj);
> +	struct iopf_group *group, *next;
> +
> +	mutex_lock(&fault->mutex);
> +	list_for_each_entry_safe(group, next, &fault->deliver, node) {
> +		list_del(&group->node);
> +		iopf_group_response(group, IOMMU_PAGE_RESP_INVALID);
> +		iopf_free_group(group);
> +	}
> +	list_for_each_entry_safe(group, next, &fault->response, node) {
> +		list_del(&group->node);
> +		device_remove_fault(group);
> +		iopf_group_response(group, IOMMU_PAGE_RESP_INVALID);
> +		iopf_free_group(group);
> +	}
> +	mutex_unlock(&fault->mutex);
> +
> +	mutex_destroy(&fault->mutex);

It is really weird to lock a mutex we are about to destroy? At this
point the refcount on the iommufd_object is zero so there had better
not be any other threads with access to this pointer!

> +static ssize_t iommufd_fault_fops_read(struct file *filep, char __user *buf,
> +				       size_t count, loff_t *ppos)
> +{
> +	size_t fault_size = sizeof(struct iommu_hwpt_pgfault);
> +	struct iommufd_fault *fault = filep->private_data;
> +	struct iommu_hwpt_pgfault data;
> +	struct iommufd_device *idev;
> +	struct iopf_group *group;
> +	struct iopf_fault *iopf;
> +	size_t done = 0;
> +	int rc;
> +
> +	if (*ppos || count % fault_size)
> +		return -ESPIPE;
> +
> +	mutex_lock(&fault->mutex);
> +	while (!list_empty(&fault->deliver) && count > done) {
> +		group = list_first_entry(&fault->deliver,
> +					 struct iopf_group, node);
> +
> +		if (list_count_nodes(&group->faults) * fault_size > count - done)
> +			break;
> +
> +		idev = (struct iommufd_device *)group->cookie->private;
> +		list_for_each_entry(iopf, &group->faults, list) {
> +			iommufd_compose_fault_message(&iopf->fault, &data, idev);
> +			rc = copy_to_user(buf + done, &data, fault_size);
> +			if (rc)
> +				goto err_unlock;
> +			done += fault_size;
> +		}
> +
> +		rc = device_add_fault(group);

See I wonder if this should be some xa_alloc or something instead of
trying to use the grpid?

> +	while (!list_empty(&fault->response) && count > done) {
> +		rc = copy_from_user(&response, buf + done, response_size);
> +		if (rc)
> +			break;
> +
> +		idev = container_of(iommufd_get_object(fault->ictx,
> +						       response.dev_id,
> +						       IOMMUFD_OBJ_DEVICE),
> +				    struct iommufd_device, obj);

It seems unfortunate we do this lookup for every iteration of the loop,
I would lift it up and cache it..

> +		if (IS_ERR(idev))
> +			break;
> +
> +		group = device_get_fault(idev, response.grpid);

This looks locked wrong, it should hold the fault mutex here and we
should probably do xchng to zero it at the same time we fetch it.

> +		if (!group ||
> +		    response.addr != group->last_fault.fault.prm.addr ||
> +		    ((group->last_fault.fault.prm.flags & IOMMU_FAULT_PAGE_REQUEST_PASID_VALID) &&
> +		      response.pasid != group->last_fault.fault.prm.pasid)) {

Why? If grpid is unique then just trust it.

> +			iommufd_put_object(fault->ictx, &idev->obj);
> +			break;
> +		}
> +
> +		iopf_group_response(group, response.code);
> +
> +		mutex_lock(&fault->mutex);
> +		list_del(&group->node);
> +		mutex_unlock(&fault->mutex);
> +
> +		device_remove_fault(group);
> +		iopf_free_group(group);
> +		done += response_size;
> +
> +		iommufd_put_object(fault->ictx, &idev->obj);
> +	}
> +
> +	return done;
> +}
> +
> +static __poll_t iommufd_fault_fops_poll(struct file *filep,
> +					struct poll_table_struct *wait)
> +{
> +	struct iommufd_fault *fault = filep->private_data;
> +	__poll_t pollflags = 0;
> +
> +	poll_wait(filep, &fault->wait_queue, wait);
> +	mutex_lock(&fault->mutex);
> +	if (!list_empty(&fault->deliver))
> +		pollflags = EPOLLIN | EPOLLRDNORM;
> +	mutex_unlock(&fault->mutex);
> +
> +	return pollflags;
> +}
> +
> +static const struct file_operations iommufd_fault_fops = {
> +	.owner		= THIS_MODULE,
> +	.open		= nonseekable_open,
> +	.read		= iommufd_fault_fops_read,
> +	.write		= iommufd_fault_fops_write,
> +	.poll		= iommufd_fault_fops_poll,
> +	.llseek		= no_llseek,
> +};

No release? That seems wrong..

> +static int get_fault_fd(struct iommufd_fault *fault)
> +{
> +	struct file *filep;
> +	int fdno;
> +
> +	fdno = get_unused_fd_flags(O_CLOEXEC);
> +	if (fdno < 0)
> +		return fdno;
> +
> +	filep = anon_inode_getfile("[iommufd-pgfault]", &iommufd_fault_fops,
> +				   fault, O_RDWR);
                                   ^^^^^^

See here you stick the iommufd_object into the FD but we don't
refcount it...

And iommufd_fault_destroy() doesn't do anything about this, so it just
UAFs the fault memory in the FD.

You need to refcount the iommufd_object here and add a release
function for the fd to put it back

*and* this FD needs to take a reference on the base iommufd_ctx fd too
as we can't tolerate them being destroyed out of sequence.

> +int iommufd_fault_alloc(struct iommufd_ucmd *ucmd)
> +{
> +	struct iommu_fault_alloc *cmd = ucmd->cmd;
> +	struct iommufd_fault *fault;
> +	int rc;
> +
> +	if (cmd->flags)
> +		return -EOPNOTSUPP;
> +
> +	fault = iommufd_object_alloc(ucmd->ictx, fault, IOMMUFD_OBJ_FAULT);
> +	if (IS_ERR(fault))
> +		return PTR_ERR(fault);
> +
> +	fault->ictx = ucmd->ictx;
> +	INIT_LIST_HEAD(&fault->deliver);
> +	INIT_LIST_HEAD(&fault->response);
> +	mutex_init(&fault->mutex);
> +	init_waitqueue_head(&fault->wait_queue);
> +
> +	rc = get_fault_fd(fault);
> +	if (rc < 0)
> +		goto out_abort;

And this is ordered wrong, fd_install has to happen after return to
user space is guarenteed as it cannot be undone.

> +	cmd->out_fault_id = fault->obj.id;
> +	cmd->out_fault_fd = rc;
> +
> +	rc = iommufd_ucmd_respond(ucmd, sizeof(*cmd));
> +	if (rc)
> +		goto out_abort;
> +	iommufd_object_finalize(ucmd->ictx, &fault->obj);

fd_install() goes here

> +	return 0;
> +out_abort:
> +	iommufd_object_abort_and_destroy(ucmd->ictx, &fault->obj);
> +
> +	return rc;
> +}

Jason