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

[Michal Hocko <mhocko@kernel.org>]

On Fri 26-04-19 16:03:26, Jann Horn wrote:
> On Fri, Apr 26, 2019 at 3:47 PM Michal Hocko <mhocko@kernel.org> wrote:
> > On Fri 26-04-19 15:33:25, Jann Horn wrote:
> > > On Fri, Apr 26, 2019 at 7:31 AM Michal Hocko <mhocko@kernel.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@kernel.org> wrote:
> > > > > [...]
> > > > > > On Wed 24-04-19 14:10:39, Matthew Garrett wrote:
> > > > > > > From: Matthew Garrett <mjg59@google.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.
> >
> > But then the VMA is gone along with the flag so why does it matter?
> 
> But in theory, the page might still be used somewhere, e.g. as data in
> a pipe (into which the parent wrote it) or whatever. Parent
> vmsplice()s a page into a pipe, parent exits, child marks the VMA as
> WIPEONRELEASE and exits, page gets wiped, someone else reads the page
> from the pipe.
> 
> Yes, this is very theoretical, and you'd have to write some pretty
> weird software for this to matter. But it doesn't seem clean to me to
> allow a child to affect the data in e.g. a pipe that it isn't supposed
> to have access to like this.
> 
> Then again, this could probably already happen, since do_wp_page()
> reuses pages depending on only the mapcount, without looking at the
> refcount.

OK, now I see your point. I was confused about the unprivileged child.
You are right that this looks weird but we have traditionally trusted
child processes to not do a harm. I guess this falls down to the same
bucket. An early CoW on these mapping should solve the problem AFAICS.

-- 
Michal Hocko
SUSE Labs