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From: Sean Christopherson <seanjc@google.com>
To: jarkko@kernel.org
Cc: linux-sgx@vger.kernel.org, dave.hansen@intel.com,
	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 09:35:28 -0800	[thread overview]
Message-ID: <YAhp4Jrj6hIcvgRC@google.com> (raw)
In-Reply-To: <20210115014638.15037-1-jarkko@kernel.org>

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
> 

  parent reply	other threads:[~2021-01-20 17:37 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 [this message]
2021-01-21  0:29   ` Jarkko Sakkinen
2021-01-21  1:19     ` Dave Hansen
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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