Re: [PATCH v5 7/8] ima: based on policy warn about loading firmware (pre-allocated buffer)

[Mimi Zohar]

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)
 Â
What about systems with an IOMMU?

Mimi