Hi,
On 1/23/25 7:48 PM, Jason Gunthorpe wrote:
> On Thu, Jan 23, 2025 at 06:10:48PM +0100, Eric Auger wrote:
>
>>> However iommufd now permits the domain to change while the driver is
>>> probed and VFIO userspace can create races with IRQ changes calling
>>> iommu_dma_prepare/compose_msi_msg() and changing/freeing the iommu_domain.
>> and is it safe in iommu_dma_prepare_msi()?
> iommu_dma_prepare_msi() takes the group mutex:
>
> int iommu_dma_prepare_msi(struct msi_desc *desc, phys_addr_t msi_addr)
> {
> struct device *dev = msi_desc_to_dev(desc);
> struct iommu_group *group = dev->iommu_group;
>
> mutex_lock(&group->mutex);
> if (group->domain && group->domain->sw_msi)
> ret = group->domain->sw_msi(group->domain, desc, msi_addr);
>
> Which prevents changing domain attachments during execution.
>
> For iommufd, if the domain attachment changes immediately after
> iommu_dma_prepare_msi() unlocks, then the information given to
> msi_desc_set_iommu_msi_iova() is still valid on the new domain.
>
> This is because the iommufd implementation of sw_msi keeps the same
> IOVA for the same ITS page globally across all domains. Any racing
> change of domain will attach a new domain with the right ITS IOVA
> already mapped and populated.
> It is why this series stops using the domain pointer as a cookie
> inside the msi_desc, immediately after the group->mutex is unlocked
> a new domain can be attached and the old domain can be freed, which
> would UAF the domain pointer in the cookie.
OK thank you for the clarification
>
>>> diff --git a/include/linux/msi.h b/include/linux/msi.h
>>> index b10093c4d00e..d442b4a69d56 100644
>>> --- a/include/linux/msi.h
>>> +++ b/include/linux/msi.h
>>> @@ -184,7 +184,8 @@ struct msi_desc {
>>> struct msi_msg msg;
>>> struct irq_affinity_desc *affinity;
>>> #ifdef CONFIG_IRQ_MSI_IOMMU
>>> - const void *iommu_cookie;
>> you may add kernel doc comments above
> I wondered if internal stuff was not being documented as the old
> iommu_cookie didn't have a comment..
>
> But sure:
>
> * @iommu_msi_iova: Optional IOVA from the IOMMU to overide the msi_addr.
> * Only used if iommu_msi_page_shift != 0
> * @iommu_msi_page_shift: Indicates how many bits of the original address
> * should be preserved when using iommu_msi_iova.
Sounds good
Eric
>
> Jason
>
