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

[Jarkko Sakkinen]

On Sat, 2021-01-30 at 21:20 +0200, Jarkko Sakkinen wrote:
> 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()?

I.e. refcount should be increased in sgx_encl_mm_add(). That way the
whole thing should be somewhat stable.

/Jarkko