linux-kernel - Re: [RFC PATCH 08/15] drivers/acrn: add VM memory management for ACRN char device

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Message-ID: <20190816124757.GW1974@kadam>
Date:   Fri, 16 Aug 2019 15:58:59 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Zhao Yakui <yakui.zhao@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        devel@...verdev.osuosl.org, Li@...osl.org, Fei <lei1.li@...el.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Liu Shuo <shuo.a.liu@...el.com>
Subject: Re: [RFC PATCH 08/15] drivers/acrn: add VM memory management for
 ACRN char device

On Fri, Aug 16, 2019 at 10:25:49AM +0800, Zhao Yakui wrote:
> +int hugepage_map_guest(struct acrn_vm *vm, struct vm_memmap *memmap)
> +{
> +	struct page *page = NULL, *regions_buf_pg = NULL;
> +	unsigned long len, guest_gpa, vma;
> +	struct vm_memory_region *region_array;
> +	struct set_regions *regions;
> +	int max_size = PAGE_SIZE / sizeof(struct vm_memory_region);
> +	int ret;
> +
> +	if (!vm || !memmap)
> +		return -EINVAL;
> +
> +	len = memmap->len;
> +	vma = memmap->vma_base;
> +	guest_gpa = memmap->gpa;
> +
> +	/* prepare set_memory_regions info */
> +	regions_buf_pg = alloc_page(GFP_KERNEL);
> +	if (!regions_buf_pg)
> +		return -ENOMEM;
> +
> +	regions = kzalloc(sizeof(*regions), GFP_KERNEL);
> +	if (!regions) {
> +		__free_page(regions_buf_pg);
> +		return -ENOMEM;

It's better to do a goto err_free_regions_buf here.  More comments
below.

> +	}
> +	regions->mr_num = 0;
> +	regions->vmid = vm->vmid;
> +	regions->regions_gpa = page_to_phys(regions_buf_pg);
> +	region_array = page_to_virt(regions_buf_pg);
> +
> +	while (len > 0) {
> +		unsigned long vm0_gpa, pagesize;
> +
> +		ret = get_user_pages_fast(vma, 1, 1, &page);
> +		if (unlikely(ret != 1) || (!page)) {
> +			pr_err("failed to pin huge page!\n");
> +			ret = -ENOMEM;
> +			goto err;

goto err is a red flag.  It's better if error labels do one specific
named thing like:

err_regions:
	kfree(regions);
err_free_regions_buf:
	__free_page(regions_buf_pg);

We should unwind in the opposite/mirror order from how things were
allocated.  Then we can remove the if statements in the error handling.

In this situation, say the user triggers an -EFAULT in
get_user_pages_fast() in the second iteration through the loop.  That
means that "page" is the non-NULL page from the previous iteration.  We
have already added it to add_guest_map().  But now we're freeing it
without removing it from the map so probably it leads to a use after
free.

The best way to write the error handling in a loop like this is to
clean up the partial iteration that has succeed (nothing here), and then
unwind all the successful iterations at the bottom of the function.
"goto unwind_loop;"

> +		}
> +
> +		vm0_gpa = page_to_phys(page);
> +		pagesize = PAGE_SIZE << compound_order(page);
> +
> +		ret = add_guest_map(vm, vm0_gpa, guest_gpa, pagesize);
> +		if (ret < 0) {
> +			pr_err("failed to add memseg for huge page!\n");
> +			goto err;

So then here, it would be:

			pr_err("failed to add memseg for huge page!\n");
			put_page(page);
			goto unwind_loop;

regards,
dan carpenter

> +		}
> +
> +		/* fill each memory region into region_array */
> +		region_array[regions->mr_num].type = MR_ADD;
> +		region_array[regions->mr_num].gpa = guest_gpa;
> +		region_array[regions->mr_num].vm0_gpa = vm0_gpa;
> +		region_array[regions->mr_num].size = pagesize;
> +		region_array[regions->mr_num].prot =
> +				(MEM_TYPE_WB & MEM_TYPE_MASK) |
> +				(memmap->prot & MEM_ACCESS_RIGHT_MASK);
> +		regions->mr_num++;
> +		if (regions->mr_num == max_size) {
> +			pr_debug("region buffer full, set & renew regions!\n");
> +			ret = set_memory_regions(regions);
> +			if (ret < 0) {
> +				pr_err("failed to set regions,ret=%d!\n", ret);
> +				goto err;
> +			}
> +			regions->mr_num = 0;
> +		}
> +
> +		len -= pagesize;
> +		vma += pagesize;
> +		guest_gpa += pagesize;
> +	}
> +
> +	ret = set_memory_regions(regions);
> +	if (ret < 0) {
> +		pr_err("failed to set regions, ret=%d!\n", ret);
> +		goto err;
> +	}
> +
> +	__free_page(regions_buf_pg);
> +	kfree(regions);
> +
> +	return 0;
> +err:
> +	if (regions_buf_pg)
> +		__free_page(regions_buf_pg);
> +	if (page)
> +		put_page(page);
> +	kfree(regions);
> +	return ret;
> +}
> +