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

[Keith Busch]

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.

> +	/* 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.