[RFC PATCH v4 7/8] ima: based on policy prevent loading firmware (pre-allocated buffer)

Ard Biesheuvel ard.biesheuvel at linaro.org
Tue Jun 5 23:20:17 PDT 2018

On 6 June 2018 at 00:37, Kees Cook <keescook at chromium.org> wrote:
> On Fri, Jun 1, 2018 at 12:25 PM, Luis R. Rodriguez <mcgrof at kernel.org> wrote:
>> On Fri, Jun 01, 2018 at 09:15:45PM +0200, Luis R. Rodriguez wrote:
>>> On Tue, May 29, 2018 at 02:01:59PM -0400, Mimi Zohar wrote:
>>> > Some systems are memory constrained but they need to load very large
>>> > firmwares.  The firmware subsystem allows drivers to request this
>>> > firmware be loaded from the filesystem, but this requires that the
>>> > entire firmware be loaded into kernel memory first before it's provided
>>> > to the driver.  This can lead to a situation where we map the firmware
>>> > twice, once to load the firmware into kernel memory and once to copy the
>>> > firmware into the final resting place.
>>> >
>>> > To resolve this problem, commit a098ecd2fa7d ("firmware: support loading
>>> > into a pre-allocated buffer") introduced request_firmware_into_buf() API
>>> > that allows drivers to request firmware be loaded directly into a
>>> > pre-allocated buffer.  The QCOM_MDT_LOADER calls dma_alloc_coherent() to
>>> > allocate this buffer.  According to Documentation/DMA-API.txt,
>>> >
>>> >      Consistent memory is memory for which a write by either the
>>> >      device or the processor can immediately be read by the processor
>>> >      or device without having to worry about caching effects.  (You
>>> >      may however need to make sure to flush the processor's write
>>> >      buffers before telling devices to read that memory.)
>>> >
>>> > Devices using pre-allocated DMA memory run the risk of the firmware
>>> > being accessible by the device prior to the kernel's firmware signature
>>> > verification has completed.
>>> Indeed. And since its DMA memory we have *no idea* what can happen in
>>> terms of consumption of this firmware from hardware, when it would start
>>> consuming it in particular.
>>> If the device has its own hardware firmware verification mechanism this is
>>> completely obscure to us, but it may however suffice certain security policies.
>>> The problem here lies in the conflicting security policies of the kernel wanting
>>> to not give away firmware until its complete and the current inability to enable
>>> us to have platforms suggest they trust hardware won't do something stupid.
>>> This becomes an issue since the semantics of the firmware API preallocated
>>> buffer do not require currently allow the kernel to inform LSMs of the fact
>>> that a buffer is DMA memory or not, and a way for certain platforms then
>>> to say that such use is fine for specific devices.
>>> Given a pointer can we determine if a piece of memory is DMA or not?
>> Vlastimil suggests page_zone() or virt_to_page() may be able to.
> I don't see a PAGEFLAG for DMA, but I do see ZONE_DMA for
> page_zone()... So maybe something like
> struct page *page;
> page = virt_to_page(address);
> if (!page)
>    fail closed...
> if (page_zone(page) == ZONE_DMA)
>     handle dma case...
> else
>     non-dma
> But I've CCed Laura and Rik, who I always lean on when I have these
> kinds of page questions...

That is not going to help. In general, DMA can access any memory in
the system (unless a IOMMU is actively preventing that).

The streaming DMA API allows you to map()/unmap() arbitrary pieces of
memory for DMA, regardless of how they were allocated. (Some drivers
were even doing DMA from the stack at some point, but this broke
vmapped stacks so most of these cases have been fixed) Uploading
firmware to a device does not require a coherent (as opposed to
streaming) mapping for DMA, and so it is perfectly reasonable for a
driver to use the streaming API to map the firmware image (wherever it
is in memory) and map it.

However, the DMA API does impose some ordering. Mapping memory for DMA
gives you a DMA address (which may be different from the physical
address [depending on the platform]), and this DMA address is what
gets programmed into the device, not the virtual or physical address.
That means you can be reasonably confident that the device will not be
able to consume what is in this memory before it has been mapped for
DMA. Also, the DMA api explicitly forbids touching memory mapped for
streaming DMA: the device owns it at this point, and so the CPU should
refrain from accessing it.

So the question is, why is QCOM_MDT_LOADER using a coherent DMA
mapping? That does not make any sense purely for moving firmware into
the device, and it is indeed a security hazard if we are trying to
perform a signature check before the device is cleared for reading it.

Note that qcom_scm_pas_init_image() is documented as

         * During the scm call memory protection will be enabled for the meta
         * data blob, so make sure it's physically contiguous, 4K aligned and
         * non-cachable to avoid XPU violations.

and dma_alloc_coherent() happens to give them that. Whether the DMA
mapping is actually used is a different matter: the code is a bit
complex, but it calls into the secure world to set up the region.

If this is the only counterexample, I wouldn't worry about it too much
(QCOM have elaborate SoC management layers in the secure world), and
simply mandate that only streaming DMA be used for firmware loading,
and that the firmware signature verification is performed before the
memory is mapped for DMA.

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