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Date:   Tue, 18 Jul 2023 07:30:11 -0700
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Subject: Re: [PATCH] x86/sgx: fix a NULL pointer
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To:     Haitao Huang <haitao.huang@linux.intel.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        dave.hansen@linux.intel.com, linux-kernel@vger.kernel.org,
        linux-sgx@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Cc:     kai.huang@intel.com, reinette.chatre@intel.com,
        kristen@linux.intel.com, seanjc@google.com, stable@vger.kernel.org
References: <CU4OBQ8MQ2LK.2GRBPLQGVTZ3@seitikki>
 <20230717202938.94989-1-haitao.huang@linux.intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20230717202938.94989-1-haitao.huang@linux.intel.com>
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On 7/17/23 13:29, Haitao Huang wrote:
...
> @@ -248,11 +258,9 @@ static struct sgx_encl_page *__sgx_encl_load_page(struct sgx_encl *encl,
>  		return entry;
>  	}
>  
> -	if (!(encl->secs.epc_page)) {
> -		epc_page = sgx_encl_eldu(&encl->secs, NULL);
> -		if (IS_ERR(epc_page))
> -			return ERR_CAST(epc_page);
> -	}
> +	epc_page = sgx_encl_load_secs(encl);
> +	if (IS_ERR(epc_page))
> +		return ERR_CAST(epc_page);
>  
>  	epc_page = sgx_encl_eldu(entry, encl->secs.epc_page);
>  	if (IS_ERR(epc_page))
> @@ -339,6 +347,13 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
>  
>  	mutex_lock(&encl->lock);
>  
> +	epc_page = sgx_encl_load_secs(encl);
> +	if (IS_ERR(epc_page)) {
> +		if (PTR_ERR(epc_page) == -EBUSY)
> +			vmret =  VM_FAULT_NOPAGE;
> +		goto err_out_unlock;
> +	}

Whenever I see one of these "make sure it isn't NULL", I always jump to
asking what *keeps* it from becoming NULL again.  In both cases here, I
think that's encl->lock.

A comment would be really nice here, maybe on sgx_encl_load_secs().  Maybe:

/*
 * Ensure the SECS page is not swapped out.  Must be called with
 * encl->lock to protect _____ and ensure the SECS page is not
 * swapped out again.
 */

> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 166692f2d501..4662a364ce62 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -257,6 +257,10 @@ static void sgx_reclaimer_write(struct sgx_epc_page *epc_page,
>  
>  	mutex_lock(&encl->lock);
>  
> +	/* Should not be possible */
> +	if (WARN_ON(!(encl->secs.epc_page)))
> +		goto out;

That comment isn't super helpful.  We generally don't WARN_ON() things
that should happen.  *Why* is it not possible?