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

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Message-ID: <20200212141029.7b89acee@lwn.net>
Date:   Wed, 12 Feb 2020 14:10:29 -0700
From:   Jonathan Corbet <corbet@....net>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, linux-api@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH] mm: extend memfd with ability to create "secret"
 memory areas

On Thu, 30 Jan 2020 18:23:41 +0200
Mike Rapoport <rppt@...nel.org> 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