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

Samiullah Khawaja
2026-03-23T19:33:58Z

On Sun, Mar 22, 2026 at 07:51:11PM +0000, Pranjal Shrivastava wrote:
>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.

Agreed. Will move the check out into the caller.
>
>> +
>> 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 */ ?

Will add.
>
>> + 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?

Thats a good point.

I think on disable/free the restored state should not be freed abruptly.
The iommu should be able to reuse the same state. We just need to mark
it as consumed/restored. I will rework the disable/free code paths to
handle this.
>
>> 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?

Yes, this needs to be checked. I will add this.
>
>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

Thanks,
Sami