linux-kernel - Re: [PATCH] mm/secretmem: use refcount_t instead of atomic

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Date:   Tue, 24 Aug 2021 17:05:50 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jordy Zomer <jordy@...ing.systems>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        James Bottomley <jejb@...ux.ibm.com>
Subject: Re: [PATCH] mm/secretmem: use refcount_t instead of atomic_t

On Thu, Aug 19, 2021 at 10:33:49PM -0700, Kees Cook wrote:
> On Fri, Aug 20, 2021 at 06:33:38AM +0200, Jordy Zomer wrote:
> > When a secret memory region is active, memfd_secret disables
> > hibernation. One of the goals is to keep the secret data from being
> > written to persistent-storage.
> > 
> > It accomplishes this by maintaining a reference count to
> > `secretmem_users`. Once this reference is held your system can not be
> > hibernated due to the check in `hibernation_available()`. However,
> > because `secretmem_users` is of type `atomic_t`, reference counter
> > overflows are possible.
> 
> It's an unlikely condition to hit given max-open-fds, etc, but there's
> no reason to leave this weakness. Changing this to refcount_t is easy
> and better than using atomic_t.
> 
> Reviewed-by: Kees Cook <keescook@...omium.org>
> 
> > As you can see there's an `atomic_inc` for each `memfd` that is opened
> > in the `memfd_secret` syscall. If a local attacker succeeds to open 2^32
> > memfd's, the counter will wrap around to 0. This implies that you may
> > hibernate again, even though there are still regions of this secret
> > memory, thereby bypassing the security check.
> 
> IMO, this hibernation check is also buggy, since it looks to be
> vulnerable to ToCToU: processes aren't frozen when
> hibernation_available() checks secretmem_users(), so a process could add
> one and fill it before the process freezer stops it.
> 
> And of course, there's still the ptrace hole[1], which is think is quite
> serious as it renders the entire defense moot.

I thought about what can be done here and could not come up with anything
better that prevent PTRACE on a process with secretmem, but this seems to
me too much from usability vs security POV.

Protecting against root is always hard and secretmem anyway does not
provide 100% guarantee by itself but rather makes an accidental data leak
or non-target attack much harder.

To be effective it also presumes that other hardening features are turned
on by the system administrator on production systems, so it's not
unrealistic to rely on ptrace being disabled.

-- 
Sincerely yours,
Mike.