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

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Date:   Wed, 24 Apr 2019 12:33:11 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Allow userland to request that the kernel clear
 memory on release

On Wed, Apr 24, 2019 at 12:28 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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?

Is there any way the page could be referenced by something other than
a VMA at this point? If so we probably don't want to zero it here, but
we do want to zero it when the page is finally released (which is why
I went with a page flag)