Re: [PATCH v5] x86/sgx: Fix use-after-free in sgx_mmu_notifier_release()

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, 2021-01-28 at 08:33 -0800, Dave Hansen wrote:
> On 1/28/21 4:58 AM, Jarkko Sakkinen wrote:
> > The most trivial example of a race condition can be demonstrated by this
> > sequence where mm_list contains just one entry:
> > 
> > CPU A                           CPU B
> > -> sgx_release()
> >                                 -> sgx_mmu_notifier_release()
> >                                 -> list_del_rcu()
> >                                 <- list_del_rcu()
> > -> kref_put()
> > -> sgx_encl_release()
> >                                 -> synchronize_srcu()
> > -> cleanup_srcu_struct()
> 
> This is missing some key details including a clear, unambiguous, problem
> statement.  To me, the patch should concentrate on the SRCU warning
> since that's where we started.  Here's the detail that needs to be added
> about the issue and the locking in general in this path:
> 
> sgx_release() also does this:
> 
>         mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
> 
> which does another synchronize_srcu() on the mmu_notifier's srcu_struct.
>  *But*, it only does this if its own list_del_rcu() is successful.  It
> does all of this before the kref_put().
> 
> In other words, sgx_release() can *only* get to this buggy path if
> sgx_mmu_notifier_release() races with sgx_release and does a
> list_del_rcu() first.
> 
> The key to this patch is that the sgx_mmu_notifier_release() will now
> take an 'encl' reference in that case, which prevents kref_put() from
> calling sgx_release() which cleans up and frees 'encl'.
> 
> I was actually also hoping to see some better comments about the new
> refcount, and the locking in general.  There are *TWO* struct_srcu's in
> play, a spinlock and a refcount.  I took me several days with Sean and
> your help to identify the actual path and get a proper fix (versions 1-4
> did *not* fix the race).

This was really good input, thank you. It made realize something but
now I need a sanity check.

I think that this bug fix is *neither* a legit one :-)

Example scenario would such that all removals "side-channel" through
the notifier callback. Then mmu_notifier_unregister() gets called
exactly zero times. No MMU notifier srcu sync would be then happening.

NOTE: There's bunch of other examples, I'm just giving one.

How I think this should be actually fixed is:

1. Whenever MMU notifier is *registered* kref_get() should be called for
   the enclave reference count.
2. *BOTH* sgx_release() and sgx_mmu_notifier_release() should
   decrease the refcount when they process an entry.
   
I.e. the fix that I sent does kref_get() in wrong location. Please
sanity check my conclusion. 
 
> Also, the use-after-free is *fixed* in sgx_mmu_notifier_release() but
> does not *occur* in sgx_mmu_notifier_release().  The subject here is a
> bit misleading in that regard.

Right, this is a valid point. It's incorrect. So if I just change the
short summary by substituting sgx_mmu_notifier_release() with
sgx_release()?

/Jarkko