Re: [PATCH v4] x86/sgx: Fix the call order of synchronize_srcu() in sgx_release()

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Jan 15, 2021 at 08:18:09AM +0100, Borislav Petkov wrote:
> On Fri, Jan 15, 2021 at 03:46:38AM +0200, jarkko@kernel.org wrote:
> > From: Jarkko Sakkinen <jarkko@kernel.org>
> > 
> > The most trivial example of a race condition can be demonstrated with this
> > example where mm_list contains just one entry:
> > 
> > CPU A                   CPU B
> > sgx_release()
> >                         sgx_mmu_notifier_release()
> >                         list_del_rcu()
> > sgx_encl_release()
> >                         synchronize_srcu()
> > cleanup_srcu_struct()
> > 
> > To fix this, call synchronize_srcu() before checking whether mm_list is
> > empty in sgx_release().
> 
> To fix what?
> 
> Lemme explain one more time: a commit message for a race condition
> fix needs to explain in *detail* what the race condition is. And that
> explanation needs to be understandable months and years from now.
> 
> You have the function call order above, now you have to explain what can
> happen. Just how you did here:
> 
> https://lkml.kernel.org/r/X/zoarV7gd/LNo4A@kernel.org

OK, I could recall the race that from but that must be partly because I've
been proactively working on it, i.e. getting your point.

So let's say I add this after the sequence:

"The sequence demonstrates a scenario where CPU B starts a new
grace period, which goes unnoticed by CPU A in sgx_release(),
because it did not remove the final entry from the enclave's
mm list."

Would this be sufficient or not?

> > Cc: stable@vger.kernel.org
> > Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Suggested-by: Haitao Huang <haitao.huang@linux.intel.com>
> > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > ---
> > v4:
> > - Rewrite the commit message.
> > - Just change the call order. *_expedited() is out of scope for this
> >   bug fix.
> > v3: Fine-tuned tags, and added missing change log for v2.
> > v2: Switch to synchronize_srcu_expedited().
> >  arch/x86/kernel/cpu/sgx/driver.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c
> > index f2eac41bb4ff..53056345f5f8 100644
> > --- a/arch/x86/kernel/cpu/sgx/driver.c
> > +++ b/arch/x86/kernel/cpu/sgx/driver.c
> > @@ -65,11 +65,16 @@ static int sgx_release(struct inode *inode, struct file *file)
> >  
> >  		spin_unlock(&encl->mm_lock);
> >  
> > +		/*
> > +		 * The call is need even if the list empty, because sgx_encl_mmu_notifier_release()
> 
> "The call is need"?
> 
> 
> $ git grep sgx_encl_mmu_notifier_release
> $
> 
> WTF?
> 
> Please be more careful.

Note taken.

> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette

/Jarkko