Re: [PATCH v8 00/15] Restricted DMA

[Claire Chang]

On Sat, Jun 5, 2021 at 1:48 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> Hi Claire,
>
> On Thu, May 27, 2021 at 08:58:30PM +0800, Claire Chang wrote:
> > This series implements mitigations for lack of DMA access control on
> > systems without an IOMMU, which could result in the DMA accessing the
> > system memory at unexpected times and/or unexpected addresses, possibly
> > leading to data leakage or corruption.
> >
> > For example, we plan to use the PCI-e bus for Wi-Fi and that PCI-e bus is
> > not behind an IOMMU. As PCI-e, by design, gives the device full access to
> > system memory, a vulnerability in the Wi-Fi firmware could easily escalate
> > to a full system exploit (remote wifi exploits: [1a], [1b] that shows a
> > full chain of exploits; [2], [3]).
> >
> > To mitigate the security concerns, we introduce restricted DMA. Restricted
> > DMA utilizes the existing swiotlb to bounce streaming DMA in and out of a
> > specially allocated region and does memory allocation from the same region.
> > The feature on its own provides a basic level of protection against the DMA
> > overwriting buffer contents at unexpected times. However, to protect
> > against general data leakage and system memory corruption, the system needs
> > to provide a way to restrict the DMA to a predefined memory region (this is
> > usually done at firmware level, e.g. MPU in ATF on some ARM platforms [4]).
> >
> > [1a] https://googleprojectzero.blogspot.com/2017/04/over-air-exploiting-broadcoms-wi-fi_4.html
> > [1b] https://googleprojectzero.blogspot.com/2017/04/over-air-exploiting-broadcoms-wi-fi_11.html
> > [2] https://blade.tencent.com/en/advisories/qualpwn/
> > [3] https://www.bleepingcomputer.com/news/security/vulnerabilities-found-in-highly-popular-firmware-for-wifi-chips/
> > [4] https://github.com/ARM-software/arm-trusted-firmware/blob/master/plat/mediatek/mt8183/drivers/emi_mpu/emi_mpu.c#L132
> >
> > v8:
> > - Fix reserved-memory.txt and add the reg property in example.
> > - Fix sizeof for of_property_count_elems_of_size in
> >   drivers/of/address.c#of_dma_set_restricted_buffer.
> > - Apply Will's suggestion to try the OF node having DMA configuration in
> >   drivers/of/address.c#of_dma_set_restricted_buffer.
> > - Fix typo in the comment of drivers/of/address.c#of_dma_set_restricted_buffer.
> > - Add error message for PageHighMem in
> >   kernel/dma/swiotlb.c#rmem_swiotlb_device_init and move it to
> >   rmem_swiotlb_setup.
> > - Fix the message string in rmem_swiotlb_setup.
>
> Thanks for the v8. It works for me out of the box on arm64 under KVM, so:
>
> Tested-by: Will Deacon <will@xxxxxxxxxx>
>
> Note that something seems to have gone wrong with the mail threading, so
> the last 5 patches ended up as a separate thread for me. Probably worth
> posting again with all the patches in one place, if you can.

Thanks for testing.

Christoph also added some comments in v7, so I'll prepare v9.

>
> Cheers,
>
> Will