Re: Why is the ARM SMMU v1/v2 put into bypass mode on kexec?
From: Robin Murphy
Date: Tue Mar 19 2024 - 08:58:08 EST
On 2024-03-14 7:06 pm, Tyler Hicks wrote:
On 2024-03-14 09:55:46, Tyler Hicks wrote:
Given that drivers are only optionally asked to implement the .shutdown
hook, which is required to properly quiesce devices before a kexec, why
is it that we put the ARM SMMU v1/v2 into bypass mode in the arm-smmu
driver's own .shutdown hook?
arm_smmu_device_shutdown() -> set SMMU_sCR0.CLIENTPD bit to 1
Driver authors often forget to even implement a .shutdown hook, which
results in some hard-to-debug memory corruption issues in the kexec'ed
target kernel due to pending DMA operations happening on untranslated
addresses. Why not leave the SMMU in translate mode but clear the stream
mapping table (or maybe even call arm_smmu_device_reset()) in the SMMU's
.shutdown hook to prevent the memory corruption from happening in the
first place?
Fully acknowledging that the proper fix is to quiesce the devices, I
feel like resetting the SMMU and leaving it in translate mode across
kexec would be more consistent with the intent behind v5.2 commit
954a03be033c ("iommu/arm-smmu: Break insecure users by disabling bypass
by default"). The incoming transactions of devices, that weren't
properly quiesced during a kexec, would be blocked until their drivers
have a chance to reinitialize the devices in the new kernel.
I appreciate any help understanding why bypass mode is utilized here as
I'm sure there are nuances that I haven't considered. Thank you!
I now see that Will has previously mentioned that he'd be open to such a
change:
One thing I would be in favour of is changing the ->shutdown() code to
honour disable_bypass=1 so that we put the SMMU in an aborting state
instead of passthrough. Would that help at all? It would at least
avoid the memory corruption on missing shutdown callback.
- https://lore.kernel.org/linux-arm-kernel/20200608113852.GA3108@willie-the-truck/
Robin mentions the need to support kexec into a non-SMMU-aware OS. I
hadn't considered that bit of complexity:
... consider if the first kernel *did* leave it enabled with whatever
left-over translations in place, and kexec'ed into another OS that
wasn't SMMU-aware...
- https://lore.kernel.org/linux-arm-kernel/e072f61a-d6cf-2e34-efd5-c1db38c5c622@xxxxxxx/
Now that we're 3-4 years removed from that conversation, has anything
changed? Will, is there anything we'd need to watch out for if we were
to prototype this sort of change? For example, would it be wise to
disable fault interrupts when putting the SMMU in an aborting state
before kexec'ing?
Fundamentally, we expect the SMMU to be disabled at initial boot, so per
the intent of kexec we put it back in that state. That also seems the
most likely expectation of anything we could kexec into, given that it
is the natural state of an untouched SMMU after a hard reset, and thus
comes out as the least-worst option.
Beyond properly quiescing and resetting the system back to a boot-time
state, the outgoing kernel in a kexec can only really do things which
affect itself. Sure, we *could* configure the SMMU to block all traffic
and disable the interrupt to avoid getting stuck in a storm of faults on
the way out, but what does that mean for the incoming kexec payload?
That it can have the pleasure of discovering the SMMU, innocently
enabling the interrupt and getting stuck in an unexpected storm of
faults. Or perhaps just resetting the SMMU into a disabled state and
thus still unwittingly allowing its memory to be corrupted by the
previous kernel not supporting kexec properly.
So no, I would not say that anything has changed here, at least not in
favour of this idea. If anything, it's become even more impractical now
that we have RMRs to properly support cases like an EFI framebuffer
where neither the outgoing nor incoming kernels necessarily have the
ability to quiesce the underlying DMA or recover it from faults, thus we
have to be even more careful.
Thanks,
Robin.