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

[Sean Christopherson <seanjc@google.com>]

On Fri, Jan 15, 2021, 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().

Why haven't you included the splat that Haitao provided?  That would go a long
way to helping answer Boris' question about exactly what is broken...

> 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()
> +		 * could have initiated a new grace period.
> +		 */
> +		synchronize_srcu(&encl->srcu);

I don't think this completely fixes the issue as sgx_release() isn't guaranteed
to trigger cleanup_srcu_struct(), e.g. the reclaimer can also have a reference
to the enclave.

> +
>  		/* The enclave is no longer mapped by any mm. */
>  		if (!encl_mm)
>  			break;
>  
> -		synchronize_srcu(&encl->srcu);
>  		mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
>  		kfree(encl_mm);
>  	}
> -- 
> 2.29.2
>