Re: [PATCH v2] iommu/arm-smmu-v3: Free MSIs in case of ENOMEM
From: Robin Murphy
Date: Tue Apr 09 2024 - 07:47:09 EST
On 09/04/2024 12:31 pm, Mostafa Saleh wrote:
Hi Robin,
On Tue, Apr 09, 2024 at 12:17:54PM +0100, Robin Murphy wrote:
On 09/04/2024 11:43 am, Mostafa Saleh wrote:
Hi Aleksandr,
On Wed, Apr 03, 2024 at 12:37:59PM +0700, Aleksandr Aprelkov wrote:
If devm_add_action() returns ENOMEM, then MSIs allocated but
not freed on teardown.
Found by Linux Verification Center (linuxtesting.org) with SVACE.
Fixes: 166bdbd23161 ("iommu/arm-smmu: Add support for MSI on SMMUv3")
Signed-off-by: Aleksandr Aprelkov <aaprelkov@xxxxxxxxxxxx>
---
v2: Use appropriate function for registration failure as
Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> suggested.
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index 41f93c3ab160..8800af041e5f 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -3402,7 +3402,9 @@ static void arm_smmu_setup_msis(struct arm_smmu_device *smmu)
smmu->priq.q.irq = msi_get_virq(dev, PRIQ_MSI_INDEX);
/* Add callback to free MSIs on teardown */
- devm_add_action(dev, arm_smmu_free_msis, dev);
+ ret = devm_add_action_or_reset(dev, arm_smmu_free_msis, dev);
+ if (ret)
+ dev_warn(dev, "failed to add free MSIs callback - falling back to wired irqs\n");
I am not sure that is the right fix, as allowing the driver to probe
without MSIs, seems worse than leaking MSI memory.
IMHO, we can just add something like:
dev_err(smmu->dev, “Can’t allocate devm action, MSIs are never freed! !\n”) ;
Honestly I don't think this matters. If we ever really did fail to allocate
16 bytes, SLUB would already be screaming and spewing stacktraces, and the
system is dead already.
Also, we can’t unconditionally fallback to wired irqs if MSI exists,
according to the user manual:
An implementation must support one of, or optionally both of,
wired interrupts and MSIs
...
The discovery of support for wired interrupts is IMPLEMENTATION DEFINED.
We can add some logic, to check dt/acpi irqs and to choose to fallback
or not based on that, but, if we get -ENOMEM, (especially early at
probe) something really went wrong, so I am not sure it’s worth
the complexity.
That logic already exists in arm_smmu_setup_unique_irqs() - the messages
here are in the sense of "we're giving up on MSIs and falling back to trying
whatever wired IRQs we may or may not have." The critical point is that
we're not using MSIs for some potentially actionable reason, i.e. if the
user does expect the system to be MSI-capable, then it could be an
indication of perhaps a wrong or missing msi-parent, for which they may
pursue a firmware fix. In other cases it's normal and expected not to use
MSIs though (e.g. the system just doesn't have an ITS), so we don't want to
be *too* noisy about it.
The case I am worried about in this patch, is for systems with
MSIs only.
With this patch, that means, we fallback to wired irqs which don't
exist, so the driver will probe with no interrupts at all, which in my
opinion worse than leaking the memory.
True, the logic looks a bit off at first glance - I was halfway through
writing a reply to that effect - but then if you look past the reality
that this is all academic since it's never really going to happen
anyway, if we *did* fail to allocate 16 bytes here, there's an
incredibly high chance that immediately proceeding into
iommu_device_sysfs_add() is also going to result in another (larger)
allocation failure which ends up aborting the whole probe anyway. Plus
the chance of subsequently being able to allocate any
domains/pagetables/etc. for any meaningful IOMMU usage would seem slim.
Honestly I'd be inclined to do nothing more than add the _or_reset to
shut the static checkers up, and not waste code and data on a useless
message for a theoretical condition at all.
Cheers,
Robin.