linux-kernel - Re: [PATCH] nvme: Cache DMA descriptors to prevent corruption.

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Message-ID: <20201120010940.GA2943603@google.com>
Date:   Thu, 19 Nov 2020 17:09:40 -0800
From:   Tom Roeder <tmroeder@...gle.com>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Peter Gonda <pgonda@...gle.com>,
        Marios Pomonis <pomonis@...gle.com>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme: Cache DMA descriptors to prevent corruption.

On Thu, Nov 19, 2020 at 01:09:14PM -0800, Keith Busch wrote:
>On Thu, Nov 19, 2020 at 10:59:19AM -0800, Tom Roeder wrote:
>> This patch changes the NVMe PCI implementation to cache host_mem_descs
>> in non-DMA memory instead of depending on descriptors stored in DMA
>> memory. This change is needed under the malicious-hypervisor threat
>> model assumed by the AMD SEV and Intel TDX architectures, which encrypt
>> guest memory to make it unreadable. Some versions of these architectures
>> also make it cryptographically hard to modify guest memory without
>> detection.
>>
>> On these architectures, Linux generally leaves DMA memory unencrypted so
>> that devices can still communicate directly with the kernel: DMA memory
>> remains readable to and modifiable by devices. This means that this
>> memory is also accessible to a hypervisor.
>>
>> However, this means that a malicious hypervisor could modify the addr or
>> size fields of descriptors and cause the NVMe driver to call
>> dma_free_attrs on arbitrary addresses or on the right addresses but with
>> the wrong size. To prevent this attack, this commit changes the code to
>> cache those descriptors in non-DMA memory and to use the cached values
>> when freeing the memory they describe.
>
>If the hypervisor does that, then the device may use the wrong
>addresses, too. I guess you can't do anything about that from the
>driver, though.
I agree; I don't think there's anything the driver can do about that.

>
>> +	/* Cache the host_mem_descs in non-DMA memory so a malicious hypervisor
>> +	 * can't change them.
>> +	 */
>> +	struct nvme_host_mem_buf_desc *host_mem_descs_cache;
>>  	void **host_mem_desc_bufs;
>
>This is never seen by an nvme device, so no need for an nvme specific
>type here. You can use arch native types.

Thanks! I'll change the type to a new struct that has the addr and size 
fields as native integers and send out a v2 for this patch that makes 
that change and cleans up a couple of minor style issues in my code.