Re: [PATCH] mm: Allow userland to request that the kernel clear memory on release

From: Matthew Wilcox <willy@infradead.org>
To: Matthew Garrett <matthewgarrett@google.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Matthew Garrett <mjg59@google.com>
Subject: Re: [PATCH] mm: Allow userland to request that the kernel clear memory on release
Date: Wed, 24 Apr 2019 12:28:12 -0700	[thread overview]
Message-ID: <20190424192812.GG19031@bombadil.infradead.org> (raw)
In-Reply-To: <20190424191440.170422-1-matthewgarrett@google.com>

On Wed, Apr 24, 2019 at 12:14:40PM -0700, Matthew Garrett wrote:
> Unfortunately, if an application exits uncleanly, its secrets may still be
> present in RAM. This can't be easily fixed in userland (eg, if the OOM
> killer decides to kill a process holding secrets, we're not going to be able
> to avoid that), so this patch adds a new flag to madvise() to allow userland
> to request that the kernel clear the covered pages whenever the page
> reference count hits zero. Since vm_flags is already full on 32-bit, it
> will only work on 64-bit systems.

Your request seems reasonable to me.

> +++ b/include/linux/page-flags.h
> @@ -118,6 +118,7 @@ enum pageflags {
>  	PG_reclaim,		/* To be reclaimed asap */
>  	PG_swapbacked,		/* Page is backed by RAM/swap */
>  	PG_unevictable,		/* Page is "unevictable"  */
> +	PG_wipeonrelease,

But you can't have a new PageFlag.  Can you instead zero the memory in
unmap_single_vma() where we call uprobe_munmap() and untrack_pfn() today?