linux-kernel - Re: [PATCH V2] mm: Allow userland to request that the kernel clear memory on release

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Date:   Fri, 26 Apr 2019 15:33:25 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Matthew Garrett <matthewgarrett@...gle.com>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH V2] mm: Allow userland to request that the kernel clear
 memory on release

On Fri, Apr 26, 2019 at 7:31 AM Michal Hocko <mhocko@...nel.org> wrote:
> On Thu 25-04-19 14:42:52, Jann Horn wrote:
> > On Thu, Apr 25, 2019 at 2:14 PM Michal Hocko <mhocko@...nel.org> wrote:
> > [...]
> > > On Wed 24-04-19 14:10:39, Matthew Garrett wrote:
> > > > From: Matthew Garrett <mjg59@...gle.com>
> > > >
> > > > Applications that hold secrets and wish to avoid them leaking can use
> > > > mlock() to prevent the page from being pushed out to swap and
> > > > MADV_DONTDUMP to prevent it from being included in core dumps. Applications
> > > > can also use atexit() handlers to overwrite secrets on application exit.
> > > > However, if an attacker can reboot the system into another OS, they can
> > > > dump the contents of RAM and extract secrets. We can avoid this by setting
> > > > CONFIG_RESET_ATTACK_MITIGATION on UEFI systems in order to request that the
> > > > firmware wipe the contents of RAM before booting another OS, but this means
> > > > rebooting takes a *long* time - the expected behaviour is for a clean
> > > > shutdown to remove the request after scrubbing secrets from RAM in order to
> > > > avoid this.
> > > >
> > > > 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.
> > [...]
> > > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > > index 21a7881a2db4..989c2fde15cf 100644
> > > > --- a/mm/madvise.c
> > > > +++ b/mm/madvise.c
> > > > @@ -92,6 +92,22 @@ static long madvise_behavior(struct vm_area_struct *vma,
> > > >       case MADV_KEEPONFORK:
> > > >               new_flags &= ~VM_WIPEONFORK;
> > > >               break;
> > > > +     case MADV_WIPEONRELEASE:
> > > > +             /* MADV_WIPEONRELEASE is only supported on anonymous memory. */
> > > > +             if (VM_WIPEONRELEASE == 0 || vma->vm_file ||
> > > > +                 vma->vm_flags & VM_SHARED) {
> > > > +                     error = -EINVAL;
> > > > +                     goto out;
> > > > +             }
> > > > +             new_flags |= VM_WIPEONRELEASE;
> > > > +             break;
> >
> > An interesting effect of this is that it will be possible to set this
> > on a CoW anon VMA in a fork() child, and then the semantics in the
> > parent will be subtly different - e.g. if the parent vmsplice()d a
> > CoWed page into a pipe, then forked an unprivileged child, the child
>
> Maybe a stupid question. How do you fork an unprivileged child (without
> exec)? Child would have to drop priviledges on its own, no?

Sorry, yes, that's what I meant.