Re: [PATCH v3] x86/sgx: Synchronize encl->srcu in sgx_encl_release().

["Haitao Huang" <haitao.huang@linux.intel.com>]

On Mon, 11 Jan 2021 18:08:10 -0600, Jarkko Sakkinen <jarkko@kernel.org>  
wrote:

> On Tue, Jan 05, 2021 at 03:57:49PM +0100, Borislav Petkov wrote:
>> On Wed, Dec 16, 2020 at 03:49:20PM +0200, Jarkko Sakkinen wrote:
>> > Add synchronize_srcu_expedited() to sgx_encl_release() to catch a  
>> grace
>> > period initiated by sgx_mmu_notifier_release().
>> >
>> > A trivial example of a failing sequence with tasks A and B:
>> >
>> > 1. A: -> sgx_release()
>> > 2. B: -> sgx_mmu_notifier_release()
>> > 3. B: -> list_del_rcu()
>> > 3. A: -> sgx_encl_release()
>> > 4. A: -> cleanup_srcu_struct()
>> >
>> > The loop in sgx_release() observes an empty list because B has  
>> removed its
>> > entry in the middle, and calls cleanup_srcu_struct() before B has a  
>> chance
>> > to calls synchronize_srcu().
>>
>> Leading to what? NULL ptr?
>>
>> https://lkml.kernel.org/r/X9e2jOWz1hfXVpQ5@google.com
>>
>> already suggested that you should explain the bug better and add the
>> splat but I'm still missing that explanation.
>
> OK, I'll try to explain it how I understand the issue.
>
> Consider this loop in the VFS release hook (sgx_release):
>
> 	/*
> 	 * Drain the remaining mm_list entries. At this point the list contains
> 	 * entries for processes, which have closed the enclave file but have
> 	 * not exited yet. The processes, which have exited, are gone from the
> 	 * list by sgx_mmu_notifier_release().
> 	 */
> 	for ( ; ; )  {
> 		spin_lock(&encl->mm_lock);
>
> 		if (list_empty(&encl->mm_list)) {
> 			encl_mm = NULL;
> 		} else {
> 			encl_mm = list_first_entry(&encl->mm_list,
> 						   struct sgx_encl_mm, list);
> 			list_del_rcu(&encl_mm->list);
> 		}
>
> 		spin_unlock(&encl->mm_lock);
>
> 		/* 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);
> 	}
>
>
> At this point all processes have closed the enclave file, but that  
> doesn't
> mean that they all have exited yet.
>
> Now, let's imagine that there is exactly one entry in the encl->mm_list.
> and sgx_release() execution gets scheduled right after returning from
> synchronize_srcu().
>
> With some bad luck, some process comes and removes that last entry befoe
> sgx_release() acquires mm_lock. The loop in sgx_release() just leaves
>
> 		/* The enclave is no longer mapped by any mm. */
> 		if (!encl_mm)
> 			break;
>
> No synchronize_srcu().
>
> After writing this, I think that the placement for synchronize_srcu()
> in this patch is not best possible. It should be rather that the
> above loop would also call synchronize_srcu() when leaving.
>
> I.e. the code change would result:
>
> 	for ( ; ; )  {
> 		spin_lock(&encl->mm_lock);
>
> 		if (list_empty(&encl->mm_list)) {
> 			encl_mm = NULL;
> 		} else {
> 			encl_mm = list_first_entry(&encl->mm_list,
> 						   struct sgx_encl_mm, list);
> 			list_del_rcu(&encl_mm->list);
> 		}
>
> 		spin_unlock(&encl->mm_lock);
>
>                 /*
>                  * synchronize_srcu() is mandatory *even* when the list  
> was
>                  * empty, in order make sure that grace periods stays in
>                  * sync even when another task took away the last entry
>                  * (i.e. exiting process when it deletes its mm_list).
>                  */
> 		synchronize_srcu(&encl->srcu);
>
> 		/* The enclave is no longer mapped by any mm. */
> 		if (!encl_mm)
> 			break;
>
> 		mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm);
> 		kfree(encl_mm);
> 	}
>
> What do you think? Does this start to make more sense now?
> I don't have logs for this but the bug can be also reasoned.
>
> /Jarkko

I did this experiment just now and find it runs much much slower than both  
original code and code with synchronize_srcu_expedited fix in this patch.
Haitao