linux-kernel - Re: [PATCH] dma-direct: Force unencrypted DMA under SME for certain DMA masks

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Date:   Wed, 24 Jul 2019 18:30:21 +0000
From:   "Lendacky, Thomas" <Thomas.Lendacky@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     Robin Murphy <robin.murphy@....com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Lianbo Jiang <lijiang@...hat.com>
Subject: Re: [PATCH] dma-direct: Force unencrypted DMA under SME for certain
 DMA masks

On 7/24/19 1:11 PM, Kirill A. Shutemov wrote:
> On Wed, Jul 24, 2019 at 05:34:26PM +0000, Lendacky, Thomas wrote:
>> On 7/24/19 12:06 PM, Robin Murphy wrote:
>>> On 24/07/2019 17:42, Lendacky, Thomas wrote:
>>>> On 7/24/19 10:55 AM, Kirill A. Shutemov wrote:
>>>>> On Wed, Jul 10, 2019 at 07:01:19PM +0000, Lendacky, Thomas wrote:
>>>>>> @@ -351,6 +355,32 @@ bool sev_active(void)
>>>>>>   }
>>>>>>   EXPORT_SYMBOL(sev_active);
>>>>>>   +/* Override for DMA direct allocation check -
>>>>>> ARCH_HAS_FORCE_DMA_UNENCRYPTED */
>>>>>> +bool force_dma_unencrypted(struct device *dev)
>>>>>> +{
>>>>>> +    /*
>>>>>> +     * For SEV, all DMA must be to unencrypted addresses.
>>>>>> +     */
>>>>>> +    if (sev_active())
>>>>>> +        return true;
>>>>>> +
>>>>>> +    /*
>>>>>> +     * For SME, all DMA must be to unencrypted addresses if the
>>>>>> +     * device does not support DMA to addresses that include the
>>>>>> +     * encryption mask.
>>>>>> +     */
>>>>>> +    if (sme_active()) {
>>>>>> +        u64 dma_enc_mask = DMA_BIT_MASK(__ffs64(sme_me_mask));
>>>>>> +        u64 dma_dev_mask = min_not_zero(dev->coherent_dma_mask,
>>>>>> +                        dev->bus_dma_mask);
>>>>>> +
>>>>>> +        if (dma_dev_mask <= dma_enc_mask)
>>>>>> +            return true;
>>>>>
>>>>> Hm. What is wrong with the dev mask being equal to enc mask? IIUC, it
>>>>> means that device mask is wide enough to cover encryption bit, doesn't it?
>>>>
>>>> Not really...  it's the way DMA_BIT_MASK works vs bit numbering. Let's say
>>>> that sme_me_mask has bit 47 set. __ffs64 returns 47 and DMA_BIT_MASK(47)
>>>> will generate a mask without bit 47 set (0x7fffffffffff). So the check
>>>> will catch anything that does not support at least 48-bit DMA.
>>>
>>> Couldn't that be expressed as just:
>>>
>>>     if (sme_me_mask & dma_dev_mask == sme_me_mask)
>>
>> Actually !=, but yes, it could have been done like that, I just didn't
>> think of it.
> 
> I'm looking into generalizing the check to cover MKTME.
> 
> Leaving	off the Kconfig changes and moving the check to other file, doest
> the change below look reasonable to you. It's only build tested so far.
> 
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index fece30ca8b0c..6c86adcd02da 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -355,6 +355,8 @@ EXPORT_SYMBOL(sev_active);
>  /* Override for DMA direct allocation check - ARCH_HAS_FORCE_DMA_UNENCRYPTED */
>  bool force_dma_unencrypted(struct device *dev)
>  {
> +	u64 dma_enc_mask;
> +
>  	/*
>  	 * For SEV, all DMA must be to unencrypted addresses.
>  	 */
> @@ -362,18 +364,20 @@ bool force_dma_unencrypted(struct device *dev)
>  		return true;
>  
>  	/*
> -	 * For SME, all DMA must be to unencrypted addresses if the
> -	 * device does not support DMA to addresses that include the
> -	 * encryption mask.
> +	 * For SME and MKTME, all DMA must be to unencrypted addresses if the
> +	 * device does not support DMA to addresses that include the encryption
> +	 * mask.
>  	 */
> -	if (sme_active()) {
> -		u64 dma_enc_mask = DMA_BIT_MASK(__ffs64(sme_me_mask));
> -		u64 dma_dev_mask = min_not_zero(dev->coherent_dma_mask,
> -						dev->bus_dma_mask);
> +	if (!sme_active() && !mktme_enabled())
> +		return false;
>  
> -		if (dma_dev_mask <= dma_enc_mask)
> -			return true;
> -	}
> +	dma_enc_mask = sme_me_mask | mktme_keyid_mask();
> +
> +	if (dev->coherent_dma_mask && (dev->coherent_dma_mask & dma_enc_mask) != dma_enc_mask)
> +		return true;
> +
> +	if (dev->bus_dma_mask && (dev->bus_dma_mask & dma_enc_mask) != dma_enc_mask)
> +		return true;

Do you want to err on the side of caution and return true if both masks
are zero? You could do the min_not_zero step and then return true if the
result is zero. Then just make the one comparison against dma_enc_mask.

Thanks,
Tom

>  
>  	return false;
>  }
>