Re: [RFC PATCH] mm: extend memfd with ability to create "secret" memory areas

[Jonathan Corbet]

On Thu, 30 Jan 2020 18:23:41 +0200
Mike Rapoport <rppt@xxxxxxxxxx> wrote:

> Hi,
> 
> This is essentially a resend of my attempt to implement "secret" mappings
> using a file descriptor [1]. 

So one little thing I was curious about as I read through the patch...

> +static int secretmem_check_limits(struct vm_fault *vmf)
> +{
> +	struct secretmem_state *state = vmf->vma->vm_file->private_data;
> +	struct inode *inode = file_inode(vmf->vma->vm_file);
> +	unsigned long limit;
> +
> +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> +		return -EINVAL;
> +
> +	limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +	if (state->nr_pages + 1 >= limit)
> +		return -EPERM;
> +
> +	return 0;
> +}

If I'm not mistaken, this means each memfd can be RLIMIT_MEMLOCK in length,
with no global limit on the number of locked pages.  What's keeping me from
creating 1000 of these things and locking down lots of RAM?

Thanks,

jon