Re: [PATCH v6 2/3] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Cyrill Gorcunov]

On Wed, Nov 09, 2022 at 03:23:02PM +0500, Muhammad Usama Anjum wrote:
...
> +
> +static long do_pagemap_sd_cmd(struct mm_struct *mm, struct pagemap_scan_arg *arg)
> +{
> +	struct mmu_notifier_range range;
> +	unsigned long __user start, end;
> +	struct pagemap_scan_private p;
> +	int ret;
> +
> +	start = (unsigned long)untagged_addr(arg->start);
> +	if ((!IS_ALIGNED(start, PAGE_SIZE)) || (!access_ok((void __user *)start, arg->len)))
> +		return -EINVAL;
> +
> +	if (IS_GET_OP(arg) &&
> +	    ((arg->vec_len == 0) || (!access_ok((struct page_region *)arg->vec, arg->vec_len))))
> +		return -ENOMEM;
> +
> +	if (IS_SD_OP(arg) && ((arg->required_mask & PAGEMAP_NONSD_OP_MASK) ||
> +	     (arg->anyof_mask & PAGEMAP_NONSD_OP_MASK)))
> +		return -EINVAL;
> +
> +	end = start + arg->len;
> +	p.max_pages = arg->max_pages;
> +	p.found_pages = 0;
> +	p.flags = arg->flags;
> +	p.required_mask = arg->required_mask;
> +	p.anyof_mask = arg->anyof_mask;
> +	p.excluded_mask = arg->excluded_mask;
> +	p.return_mask = arg->return_mask;
> +	p.vec_index = 0;
> +	p.vec_len = arg->vec_len;
> +
> +	if (IS_GET_OP(arg)) {
> +		p.vec = vzalloc(arg->vec_len * sizeof(struct page_region));
> +		if (!p.vec)
> +			return -ENOMEM;
> +	} else {
> +		p.vec = NULL;
> +	}

Hi Muhammad! I'm really sorry for diving in such late (unfortunatelly too busy to
step in yet). Anyway, while in general such interface looks reasonable here are
few moments which really bothers me: as far as I undertstand you don't need
vzalloc here, plain vmalloc should works as well since you copy only filled
results back to userspace. Next -- there is no restriction on vec_len parameter,
is not here a door for DoS from userspace? Say I could start a number of ioctl
on same pagemap and try to allocate very big amount of vec_len in summay causing
big pressure on kernel's memory. Or I miss something obvious here?