Re: [PATCH rc 1/2] iommu: Do not call drivers for empty gathers

Robin Murphy
2026-03-03T12:53:37Z

On 2026-03-02 10:22 pm, Jason Gunthorpe wrote:
> An empty gather is coded with start=U64_MAX, end=0 and several drivers go
> on to convert that to a size with:
>
> end - start + 1
>
> Which gives 2 for an empty gather. This then causes Weird Stuff to
> happen (for example an UBSAN splat in VT-d) that is hopefully harmless,
> but maybe not.
>
> Prevent drivers from being called right in iommu_iotlb_sync().
>
> Auditing shows that AMD, Intel, Mediatek and RSIC-V drivers all do things
> on these empty gathers.
>
> Further, there are several callers that can trigger empty gathers,
> especially in unusual conditions. For example iommu_map_nosync() will call
> a 0 size unmap on some error paths. Also in VFIO, iommupt and other
> places.

My instinct is still to tidy up the 0-length unmap case(s), but I guess
iommu_iotlb_sync() is itself also a public API where being more robust
against erroneous usage is no bad thing. With one minor nit below,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Cc: stable@vger.kernel.org
> Reported-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
> Closes: https://lore.kernel.org/r/11145826.aFP6jjVeTY@jkrzyszt-mobl2.ger.corp.intel.com
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> include/linux/iommu.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 54b8b48c762e88..555597b54083cd 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -980,7 +980,8 @@ static inline void iommu_flush_iotlb_all(struct iommu_domain *domain)
> static inline void iommu_iotlb_sync(struct iommu_domain *domain,
> struct iommu_iotlb_gather *iotlb_gather)
> {
> - if (domain->ops->iotlb_sync)
> + if (domain->ops->iotlb_sync &&
> + likely(iotlb_gather->start < iotlb_gather->end))

Elsewhere we just use "gather->end != 0" as the "non-empty" condition;
how concerned are we about defending against more-intentionally
malformed gathers here?

Thanks,
Robin.

> domain->ops->iotlb_sync(domain, iotlb_gather);
>
> iommu_iotlb_gather_init(iotlb_gather);