The concept of IOMMUs being in strict vs. non-strict mode is a pre-existing Linux concept. I've included a rough summary here to help evaluate this patch.
IOMMUs can be run in "strict" mode or in "non-strict" mode. The quick-summary difference between the two is that in "strict" mode we wait until everything is flushed out when we unmap DMA memory. In "non-strict" we don't. Using the IOMMU in "strict" mode is more secure/safer but slower because we have to sit and wait for flushes while we're unmapping. To explain a bit why "non-strict" mode is unsafe, let's imagine two examples. An example of "non-strict" being insecure when reading from a device: a) Linux driver maps memory for DMA. b) Linux driver starts DMA on the device. c) Device write to RAM subject to bounds checking done by IOMMU. d) Device finishes writing to RAM and signals transfer is finished. e) Linux driver starts unmapping DMA memory but doesn't wait for the unmap to finish (the definition of non-strict). At this point, though, the Linux APIs say that the driver owns the memory and shouldn't expect any more scribbling from the DMA device. f) Linux driver validates that the data in memory looks sane and that accessing it won't cause the driver to, for instance, overflow a buffer. g) Device takes advantage of knowledge of how the Linux driver works and sneaks in a modification to the data after the validation but before the IOMMU unmap flush finishes. h) Device has now caused the Linux driver to access memory it shouldn't. An example of "non-strict" being insecure when writing to a device: a) Linux driver writes data intended for the device to RAM. b) Linux driver maps memory for DMA. c) Linux driver starts DMA on the device. d) Device reads from RAM subject to bounds checking done by IOMMU. e) Device finishes reading from RAM and signals transfer is finished. f) Linux driver starts unmapping DMA memory but doesn't wait for the unmap to finish (the definition of non-strict) g) Linux driver frees memory and returns it to the pool of memory available for other users to allocate. h) Memory is allocated for another purpose since it was free memory. i) Device takes advantage of the period of time before IOMMU flush to read memory that it shouldn't have had access to. What exactly the memory could contain depends on the randomness of who allocated next, though exploits have been built on flimisier holes. As you can see from the above examples, using the iommu in "non-strict" mode might not sound _too_ scary (the window of badness is small and the exposed memory is small) but there is certainly risk. Let's evaluate the risk by breaking it down into two problems that IOMMUs are supposed to be protecting us against: Case 1: IOMMUs prevent malicious code running on the peripheral (maybe a malicious peripheral or maybe someone exploited a benign peripheral) from turning into an exploit of the Linux kernel. This is particularly important if the peripheral has loadable / updatable firmware or if the peripheral has some type of general purpose processor and is processing untrusted inputs. It's also important if the device is something that can be easily plugged into the host and the device has direct DMA access itself, like a PCIe device. Case 2: IOMMUs limit the severity of a class of software bugs in the kernel. If we misconfigure a peripheral by accident then instead of the peripheral clobbering random memory due to a bug we might get an IOMMU error. Now that we understand the issue and the risks, let's evaluate whether we really need "strict" mode for the Qualcomm SDHCI controllers. I will make the argument that we don't _need_ strict mode for them. Why? * The SDHCI controller on Qualcomm SoCs doesn't appear to have loadable / updatable firmware and, assuming it's got some firmware baked into it, I see no evidence that the firmware could be compromised. * Even though, for external SD cards in particular, the controller is dealing with "untrusted" inputs, it's dealing with them in a very controlled way. It seems unlikely that a rogue SD card would be able to present something to the SDHCI controller that would cause it to DMA to/from an address other than one the kernel told it about. * Although it would be nice to catch more software bugs, once the Linux driver has been debugged and stressed the value is not very high. If the IOMMU caught something like this the system would be in a pretty bad shape anyway (we don't really recover from IOMMU errors) and the only benefit would be a better spotlight on what went wrong. Now we have a good understanding of the benefits of "strict" mode for our SDHCI controllers, let's look at some performance numbers. I used "dd" to measure read speeds from eMMC on a sc7180-trogdor-lazor board. Basic test command (while booted from USB): echo 3 > /proc/sys/vm/drop_caches dd if=/dev/mmcblk1 of=/dev/null bs=4M count=512 I attempted to run my tests for enough iterations that results stabilized and weren't too noisy. Tests were run with patches picked to the chromeos-5.4 tree (sanity checked against v5.13-rc7). I also attempted to compare to other attempts to address IOMMU problems and/or attempts to bump the cpufreq up to solve this problem: - eMMC datasheet spec: 300 MB/s "Typical Sequential Performance" NOTE: we're driving the bus at 192 MHz instead of 200 Mhz so we might not be able to achieve the full 300 MB/s. - Baseline: 210.9 MB/s - Baseline + peg cpufreq to max: 284.3 MB/s - This patch: 279.6 MB/s - This patch + peg cpufreq to max: 288.1 MB/s - Joel's IO Wait fix [1]: 258.4 MB/s - Joel's IO Wait fix [1] + peg cpufreq to max: 287.8 MB/s - TLBIVA patches [2] + [3]: 214.7 MB/s - TLBIVA patches [2] + [3] + peg cpufreq to max: 285.7 MB/s - This patch plus Joel's [1]: 280.2 MB/s - This patch plus Joel's [1] + peg...: 279.0 MB/s NOTE: I suspect something in the system was thermal throttling since there's a heat wave right now. I also spent a little bit of time trying to see if I could get the IOMMU flush for MMC out of the critical path but was unable to figure out how to do this and get good performance. Overall I'd say that the performance results above show: * It's really not straightforward to point at "one thing" that is making our eMMC performance bad. * It's certainly possible to get pretty good eMMC performance even without this patch. * This patch makes it much easier to get good eMMC performance. * No other solutions that I found resulted in quite as good eMMC performance as having this patch. Given all the above (security safety concerns are minimal and it's a nice performance win), I'm proposing that running SDHCI on Qualcomm SoCs in non-strict mode is the right thing to do until such point in time as someone can come up with a better solution to get good SD/eMMC performance without it. Now that we've decided we want the SD/MMC controller in non-strict mode, we need to figure out how to make it happen. We will take advantage of the fact that on Qualcomm IOMMUs we know that SD/MMC controllers are in a domain by themselves and hook in when initting the domain context. In response to a previous version of this series there had been discussion [4] of having this driven from a device tree property and this solution doesn't preclude that but is a good jumping off point. NOTES: * It's likely that arguments similar to the above can be made for other SDHCI controllers. However, given that this is something that can have an impact on security it feels like we want each SDHCI controller to opt-in. I believe it is conceivable, for instance, that some SDHCI controllers might have loadable or updatable firmware. * It's also likely other peripherals will want this to get the quick performance win. That also should be fine, though anyone landing a similar patch should be very careful that it is low risk for all users of a given peripheral. * Conceivably if even this patch is considered too "high risk", we could limit this to just non-removable cards (like eMMC) by just checking the device tree. This is one nice advantage of using the pre_probe() to set this. [1] https://lore.kernel.org/r/20210618040639.3113489-1-j...@joelfernandes.org [2] https://lore.kernel.org/r/1623850736-389584-1-git-send-email-quic_c_gdj...@quicinc.com/ [3] https://lore.kernel.org/r/cover.1623981933.git.saiprakash.ran...@codeaurora.org/ [4] https://lore.kernel.org/r/20210621235248.2521620-1-diand...@chromium.org Signed-off-by: Douglas Anderson <diand...@chromium.org> --- Changes in v2: - Now accomplish the goal by putting rules in the IOMMU driver. - Reworded commit message to clarify things pointed out by Greg. drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c index 98b3a1c2a181..bd66376d21ce 100644 --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c @@ -172,6 +172,24 @@ static const struct of_device_id qcom_smmu_client_of_match[] __maybe_unused = { { } }; +static const struct of_device_id qcom_smmu_nonstrict_of_match[] __maybe_unused = { + { .compatible = "qcom,sdhci-msm-v4" }, + { .compatible = "qcom,sdhci-msm-v5" }, + { } +}; + +static int qcom_smmu_init_context(struct arm_smmu_domain *smmu_domain, + struct io_pgtable_cfg *pgtbl_cfg, struct device *dev) +{ + const struct of_device_id *match = + of_match_device(qcom_smmu_nonstrict_of_match, dev); + + if (match) + smmu_domain->domain.strictness = IOMMU_NOT_STRICT; + + return 0; +} + static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu) { unsigned int last_s2cr = ARM_SMMU_GR0_S2CR(smmu->num_mapping_groups - 1); @@ -295,6 +313,7 @@ static int qcom_smmu500_reset(struct arm_smmu_device *smmu) } static const struct arm_smmu_impl qcom_smmu_impl = { + .init_context = qcom_smmu_init_context, .cfg_probe = qcom_smmu_cfg_probe, .def_domain_type = qcom_smmu_def_domain_type, .reset = qcom_smmu500_reset, -- 2.32.0.93.g670b81a890-goog _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
