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From: Dave Hansen <dave.hansen@intel.com>
To: Jarkko Sakkinen <jarkko@kernel.org>,
	Sean Christopherson <seanjc@google.com>
Cc: linux-sgx@vger.kernel.org, kai.huang@intel.com,
	haitao.huang@intel.com, stable@vger.kernel.org,
	Haitao Huang <haitao.huang@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Jethro Beekman <jethro@fortanix.com>
Subject: Re: [PATCH v4] x86/sgx: Fix the call order of synchronize_srcu() in sgx_release()
Date: Wed, 20 Jan 2021 17:19:44 -0800	[thread overview]
Message-ID: <042741ff-1436-f3f2-df23-a524d1d7026d@intel.com> (raw)
In-Reply-To: <YAjK9n6zct2VTp74@kernel.org>

On 1/20/21 4:29 PM, Jarkko Sakkinen wrote:
> On Wed, Jan 20, 2021 at 09:35:28AM -0800, Sean Christopherson wrote:
>> 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...
> I've lost the klog.

Haitao said he thought it was this:

> void cleanup_srcu_struct(struct srcu_struct *ssp)
> {
...
>         if (WARN_ON(srcu_readers_active(ssp)))
>                 return; /* Just leak it! */

Which would indicate that an 'encl' kref reached 0 while some other
thread was inside a

        idx = srcu_read_lock(&encl->srcu);
	...
	srcu_read_unlock(&encl->srcu, idx);

critical section.  A quick audit didn't turn up any obvious suspects,
though.

If that *is* it, it might be nice to try to catch the culprit at
srcu_read_{un}lock() time.  If there's ever a 0 refcount at those sites,
it would be nice to spew a warning:

        idx = srcu_read_lock(&encl->srcu);
	WARN_ON(!atomic_read(&encl->refcount->refcount);
	...
	WARN_ON(!atomic_read(&encl->refcount->refcount);
	srcu_read_unlock(&encl->srcu, idx);

at each site.

  reply	other threads:[~2021-01-21  1:47 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-15  1:46 [PATCH v4] x86/sgx: Fix the call order of synchronize_srcu() in sgx_release() jarkko
2021-01-15  7:18 ` Borislav Petkov
2021-01-16  5:12   ` Jarkko Sakkinen
2021-01-18 18:57     ` Borislav Petkov
2021-01-20 14:43       ` Jarkko Sakkinen
2021-01-20 17:34         ` Dave Hansen
2021-01-21  0:26           ` Jarkko Sakkinen
2021-01-22 18:20             ` Haitao Huang
2021-01-20 17:35 ` Sean Christopherson
2021-01-21  0:29   ` Jarkko Sakkinen
2021-01-21  1:19     ` Dave Hansen [this message]
2021-01-21 12:55       ` Jarkko Sakkinen
2021-01-21 18:19         ` Dave Hansen
2021-01-22 16:56   ` Dave Hansen
2021-01-23  8:58     ` Jarkko Sakkinen
2021-01-25 15:49       ` Dave Hansen
2021-01-27 17:31         ` Jarkko Sakkinen

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