On 9 July 2018 at 21:41, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> On Mon, 2018-07-02 at 17:30 +0200, Ard Biesheuvel wrote:
>> On 2 July 2018 at 16:38, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> 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. (Based on the mailing list discussions, calling
>> > dma_alloc_coherent() is unnecessary and confusing.)
>> >
>> > (Very broken/buggy) devices using pre-allocated memory run the risk of
>> > the firmware being accessible to the device prior to the completion of
>> > IMA's signature verification. For the time being, this patch emits a
>> > warning, but does not prevent the loading of the firmware.
>> >
>>
>> As I attempted to explain in the exchange with Luis, this has nothing
>> to do with broken or buggy devices, but is simply the reality we have
>> to deal with on platforms that lack IOMMUs.
>
>> Even if you load into one buffer, carry out the signature verification
>> and *only then* copy it to another buffer, a bus master could
>> potentially read it from the first buffer as well. Mapping for DMA
>> does *not* mean 'making the memory readable by the device' unless
>> IOMMUs are being used. Otherwise, a bus master can read it from the
>> first buffer, or even patch the code that performs the security check
>> in the first place. For such platforms, copying the data around to
>> prevent the device from reading it is simply pointless, as well as any
>> other mitigation in software to protect yourself from misbehaving bus
>> masters.
>
> Thank you for taking the time to explain this again.
>
>> So issuing a warning in this particular case is rather arbitrary. On
>> these platforms, all bus masters can read (and modify) all of your
>> memory all of the time, and as long as the firmware loader code takes
>> care not to provide the DMA address to the device until after the
>> verification is complete, it really has done all it reasonably can in
>> the environment that it is expected to operate in.
>
> So for the non-IOMMU system case, differentiating between pre-
> allocated buffers vs. using two buffers doesn't make sense.
>
>>
>> (The use of dma_alloc_coherent() is a bit of a red herring here, as it
>> incorporates the DMA map operation. However, DMA map is a no-op on
>> systems with cache coherent 1:1 DMA [iow, all PCs and most arm64
>> platforms unless they have IOMMUs], and so there is not much
>> difference between memory allocated with kmalloc() or with
>> dma_alloc_coherent() in terms of whether the device can access it
>> freely)
>
> What about systems with an IOMMU?
>
On systems with an IOMMU, performing the DMA map will create an entry
in the IOMMU page tables for the physical region associated with the
buffer, making the region accessible to the device. For platforms in
this category, using dma_alloc_coherent() for allocating a buffer to
pass firmware to the device does open a window where the device could
theoretically access this data while the validation is still in
progress.
Note that the device still needs to be informed about the address of
the buffer: just calling dma_alloc_coherent() will not allow the
device to find the firmware image in its memory space, and arbitrary
DMA accesses performed by the device will trigger faults that are
reported to the OS. So the window between DMA map (or
dma_alloc_coherent()) and the device specific command to pass the DMA
buffer address to the device is not inherently unsafe IMO, but I do
understand the need to cover this scenario.
As I pointed out before, using coherent DMA buffers to perform
streaming DMA is generally a bad idea, since they may be allocated
from a finite pool, and may use uncached mappings, making the access
slower than necessary (while streaming DMA can use any kmalloc'ed
buffer and will just flush the contents of the caches to main memory
when the DMA map is performed).
So to summarize again: in my opinion, using a single buffer is not a
problem as long as the validation completes before the DMA map is
performed. This will provide the expected guarantees on systems with
IOMMUs, and will not complicate matters on systems where there is no
point in obsessing about this anyway given that devices can access all
of memory whenever they want to.
As for the Qualcomm case: dma_alloc_coherent() is not needed here but
simply ends up being used because it was already wired up in the
qualcomm specific secure world API, which amounts to doing syscalls
into a higher privilege level than the one the kernel itself runs at.
So again, reasoning about whether the secure world will look at your
data before you checked the sig is rather pointless, and adding
special cases to the IMA api to cater for this use case seems like a
waste of engineering and review effort to me. If we have to do
something to tie up this loose end, let's try switching it to the
streaming DMA api instead.
--
Ard.
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