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Date:   Sun, 16 Feb 2020 08:46:50 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     Jonathan Corbet <corbet@lwn.net>
Cc:     linux-kernel@vger.kernel.org, Alan Cox <alan@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Christopher Lameter <cl@linux.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        James Bottomley <jejb@linux.ibm.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tycho Andersen <tycho@tycho.ws>, linux-api@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [RFC PATCH] mm: extend memfd with ability to create "secret"
 memory areas
Message-ID: <20200216064650.GB22092@hump.haifa.ibm.com>
References: <20200130162340.GA14232@rapoport-lnx>
 <20200212141029.7b89acee@lwn.net>
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On Wed, Feb 12, 2020 at 02:10:29PM -0700, Jonathan Corbet wrote:
> On Thu, 30 Jan 2020 18:23:41 +0200
> Mike Rapoport <rppt@kernel.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?

Indeed, it's possible to lock down RLIMIT_MEMLOCK * RLIMIT_NOFILE of RAM
with this implementation, thanks for catching this.

I'll surely update the resource limiting once we've settle on the API
selection :)
 
> Thanks,
> 
> jon
> 

-- 
Sincerely yours,
Mike.