Re: [PATCH 07/14] iommu/vt-d: Restore IOMMU state and reclaimed domain ids

[Pranjal Shrivastava <praan@google.com>]

On Tue, Feb 03, 2026 at 10:09:41PM +0000, Samiullah Khawaja wrote:
> During boot fetch the preserved state of IOMMU unit and if found then
> restore the state.
> 
> - Reuse the root_table that was preserved in the previous kernel.
> - Reclaim the domain ids of the preserved domains for each preserved
>   devices so these are not acquired by another domain.
> 
> Signed-off-by: Samiullah Khawaja <skhawaja@google.com>
> ---
>  drivers/iommu/intel/iommu.c      | 26 +++++++++++++++------
>  drivers/iommu/intel/iommu.h      |  7 ++++++
>  drivers/iommu/intel/liveupdate.c | 40 ++++++++++++++++++++++++++++++++
>  3 files changed, 66 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index c95de93fb72f..8acb7f8a7627 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -222,12 +222,12 @@ static void clear_translation_pre_enabled(struct intel_iommu *iommu)
>  	iommu->flags &= ~VTD_FLAG_TRANS_PRE_ENABLED;
>  }
>  
> -static void init_translation_status(struct intel_iommu *iommu)
> +static void init_translation_status(struct intel_iommu *iommu, bool restoring)
>  {
>  	u32 gsts;
>  
>  	gsts = readl(iommu->reg + DMAR_GSTS_REG);
> -	if (gsts & DMA_GSTS_TES)
> +	if (!restoring && (gsts & DMA_GSTS_TES))
>  		iommu->flags |= VTD_FLAG_TRANS_PRE_ENABLED;
>  }
>  
> @@ -670,10 +670,16 @@ void dmar_fault_dump_ptes(struct intel_iommu *iommu, u16 source_id,
>  #endif
>  
>  /* iommu handling */
> -static int iommu_alloc_root_entry(struct intel_iommu *iommu)
> +static int iommu_alloc_root_entry(struct intel_iommu *iommu, struct iommu_ser *restored_state)
>  {
>  	struct root_entry *root;
>  
> +	if (restored_state) {
> +		intel_iommu_liveupdate_restore_root_table(iommu, restored_state);
> +		__iommu_flush_cache(iommu, iommu->root_entry, ROOT_SIZE);
> +		return 0;
> +	}

Instead of putting this inside the allocator, shouldn't init_dmars and
intel_iommu_add check for iommu_ser and call 
intel_iommu_liveupdate_restore_root_table() directly, bypassing the 
allocation entirely? This looks like it could be a stand-alone function
which has nothing to do with allocation.

> +
>  	root = iommu_alloc_pages_node_sz(iommu->node, GFP_ATOMIC, SZ_4K);
>  	if (!root) {
>  		pr_err("Allocating root entry for %s failed\n",
> @@ -1614,6 +1620,7 @@ static int copy_translation_tables(struct intel_iommu *iommu)
>  
>  static int __init init_dmars(void)
>  {
> +	struct iommu_ser *iommu_ser = NULL;
>  	struct dmar_drhd_unit *drhd;
>  	struct intel_iommu *iommu;
>  	int ret;
> @@ -1636,8 +1643,10 @@ static int __init init_dmars(void)
>  						   intel_pasid_max_id);
>  		}
>  
> +		iommu_ser = iommu_get_preserved_data(iommu->reg_phys, IOMMU_INTEL);
> +
>  		intel_iommu_init_qi(iommu);
> -		init_translation_status(iommu);
> +		init_translation_status(iommu, !!iommu_ser);
>  
>  		if (translation_pre_enabled(iommu) && !is_kdump_kernel()) {
>  			iommu_disable_translation(iommu);
> @@ -1651,7 +1660,7 @@ static int __init init_dmars(void)
>  		 * we could share the same root & context tables
>  		 * among all IOMMU's. Need to Split it later.
>  		 */
> -		ret = iommu_alloc_root_entry(iommu);
> +		ret = iommu_alloc_root_entry(iommu, iommu_ser);
>  		if (ret)
>  			goto free_iommu;
>  
> @@ -2110,15 +2119,18 @@ int dmar_parse_one_satc(struct acpi_dmar_header *hdr, void *arg)
>  static int intel_iommu_add(struct dmar_drhd_unit *dmaru)
>  {
>  	struct intel_iommu *iommu = dmaru->iommu;
> +	struct iommu_ser *iommu_ser = NULL;
>  	int ret;
>  

Nit: Add: /* Fetch the preserved context using MMIO base as a token */ ?

> +	iommu_ser = iommu_get_preserved_data(iommu->reg_phys, IOMMU_INTEL);
> +
>  	/*
>  	 * Disable translation if already enabled prior to OS handover.
>  	 */
> -	if (iommu->gcmd & DMA_GCMD_TE)
> +	if (!iommu_ser && iommu->gcmd & DMA_GCMD_TE)
>  		iommu_disable_translation(iommu);
>  
> -	ret = iommu_alloc_root_entry(iommu);
> +	ret = iommu_alloc_root_entry(iommu, iommu_ser);

I understand that iommu_get_preserved_data() will eventually return NULL
after the flb_finish op has executed (based on the LUO IOCTLs dropping
the incoming state), but I'm sensing a potential UAF/double-restore
issue here that could happen during the boot window.

I believe we could restore the same context multiple times? I see 
intel_iommu_add() is called from both dmar_device_add() and 
dmar_device_remove() paths, and the ACPI probe has the following 
sequence [1]:

static int acpi_pci_root_add(struct acpi_device *device, ...)
{
	// ...
	if (hotadd && dmar_device_add(handle)) {
		result = -ENXIO;
		goto end;
	}

	// ...
	root->bus = pci_acpi_scan_root(root);
	if (!root->bus) {
		// ...
		result = -ENODEV;
		goto remove_dmar;
	}
	// ...

remove_dmar:
	if (hotadd)
		dmar_device_remove(handle);
end:
	return result;
}

If we successfully restored a domain during dmar_device_add(), but the
ACPI probe fails later (e.g., pci_acpi_scan_root fails), we jump to 
remove_dmar. This tears down the DMAR unit, it unwinds via 
dmar_device_remove() which eventually calls dmar_iommu_hotplug(false)
where we:

	disable_dmar_iommu(iommu);
	free_dmar_iommu(iommu);

At this point, the root table folios are freed back to the allocator.

However, if a re-scan is then triggered before the FLB drops the 
incoming state, we would call:

dmar_device_add() -> intel_iommu_add() -> iommu_alloc_root_entry() again

Because the KHO state wasn't marked as deleted/consumed, 
iommu_get_preserved_data() will hand us the exact same iommu_ser pointer?

In which case, we'd call kho_restore_folio(iommu_ser->intel.root_table)
on a physical page that might have already been reallocated?

Shouldn't the restored state be explicitly marked as consumed
(obj.deleted = 1), and shouldn't the driver properly unpreserve/clean up
the KHO tracking during the free_dmar_iommu() teardown path?

>  	if (ret)
>  		goto out;
>  
> diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
> index 70032e86437d..d7bf63aff17d 100644
> --- a/drivers/iommu/intel/iommu.h
> +++ b/drivers/iommu/intel/iommu.h
> @@ -1283,6 +1283,8 @@ int intel_iommu_preserve_device(struct device *dev, struct device_ser *device_se
>  void intel_iommu_unpreserve_device(struct device *dev, struct device_ser *device_ser);
>  int intel_iommu_preserve(struct iommu_device *iommu, struct iommu_ser *iommu_ser);
>  void intel_iommu_unpreserve(struct iommu_device *iommu, struct iommu_ser *iommu_ser);
> +void intel_iommu_liveupdate_restore_root_table(struct intel_iommu *iommu,
> +					       struct iommu_ser *iommu_ser);
>  #else
>  static inline int intel_iommu_preserve_device(struct device *dev, struct device_ser *device_ser)
>  {
> @@ -1301,6 +1303,11 @@ static inline int intel_iommu_preserve(struct iommu_device *iommu, struct iommu_
>  static inline void intel_iommu_unpreserve(struct iommu_device *iommu, struct iommu_ser *iommu_ser)
>  {
>  }
> +
> +static inline void intel_iommu_liveupdate_restore_root_table(struct intel_iommu *iommu,
> +							     struct iommu_ser *iommu_ser)
> +{
> +}
>  #endif
>  
>  #ifdef CONFIG_INTEL_IOMMU_SVM
> diff --git a/drivers/iommu/intel/liveupdate.c b/drivers/iommu/intel/liveupdate.c
> index 82ba1daf1711..6dcb5783d1db 100644
> --- a/drivers/iommu/intel/liveupdate.c
> +++ b/drivers/iommu/intel/liveupdate.c
> @@ -73,6 +73,46 @@ static int preserve_iommu_context(struct intel_iommu *iommu)
>  	return ret;
>  }
>  
> +static void restore_iommu_context(struct intel_iommu *iommu)
> +{
> +	struct context_entry *context;
> +	int i;
> +
> +	for (i = 0; i < ROOT_ENTRY_NR; i++) {
> +		context = iommu_context_addr(iommu, i, 0, 0);
> +		if (context)
> +			BUG_ON(!kho_restore_folio(virt_to_phys(context)));
> +
> +		if (!sm_supported(iommu))
> +			continue;
> +
> +		context = iommu_context_addr(iommu, i, 0x80, 0);
> +		if (context)
> +			BUG_ON(!kho_restore_folio(virt_to_phys(context)));
> +	}
> +}
> +
> +static int __restore_used_domain_ids(struct device_ser *ser, void *arg)
> +{
> +	int id = ser->domain_iommu_ser.did;
> +	struct intel_iommu *iommu = arg;
> +

Shouldn't we check if the did actually belongs to the iommu instance?
iommu_for_each_preserved_device() iterates over all preserved devices in
the system. However, here (__restore_used_domain_ids) we allocate the 
device's did in the current iommu->domain_ida without checking if that
device actually belongs to the current IOMMU?

On multi-IOMMU systems, this will cause every IOMMU's IDA to be
cross-polluted with the domain IDs of devices attached to other IOMMUs.
We must verify the device belongs to this specific IOMMU first, maybe:

if (ser->domain_iommu_ser.iommu_phys != iommu->reg_phys)
		return 0;

> +	ida_alloc_range(&iommu->domain_ida, id, id, GFP_ATOMIC);
> +	return 0;
> +}
> +
> +void intel_iommu_liveupdate_restore_root_table(struct intel_iommu *iommu,
> +					       struct iommu_ser *iommu_ser)
> +{
> +	BUG_ON(!kho_restore_folio(iommu_ser->intel.root_table));
> +	iommu->root_entry = __va(iommu_ser->intel.root_table);
> +
> +	restore_iommu_context(iommu);
> +	iommu_for_each_preserved_device(__restore_used_domain_ids, iommu);
> +	pr_info("Restored IOMMU[0x%llx] Root Table at: 0x%llx\n",
> +		iommu->reg_phys, iommu_ser->intel.root_table);
> +}
> +
>  int intel_iommu_preserve_device(struct device *dev, struct device_ser *device_ser)
>  {
>  	struct device_domain_info *info = dev_iommu_priv_get(dev);

Thanks,
Praan

[1] https://elixir.bootlin.com/linux/v7.0-rc4/source/drivers/acpi/pci_root.c#L728