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

Cyrill Gorcunov
2022-12-12T20:42:24Z

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?