On 2026-03-16 12:58 pm, Jiri Pirko wrote:

From: Jiri Pirko jiri@nvidia.com

Current CC designs don't place a vIOMMU in front of untrusted devices. Instead, the DMA API forces all untrusted device DMA through swiotlb bounce buffers (is_swiotlb_force_bounce()) which copies data into decrypted memory on behalf of the device.

When a caller has already arranged for the memory to be decrypted via set_memory_decrypted(), the DMA API needs to know so it can map directly using the unencrypted physical address rather than bounce buffering. Following the pattern of DMA_ATTR_MMIO, add DMA_ATTR_CC_DECRYPTED for this purpose. Like the MMIO case, only the caller knows what kind of memory it has and must inform the DMA API for it to work correctly.

Echoing Jason's point, if the intent of this is to indicate shared memory, please call it DMA_ATTR_CC_SHARED. Yes, some of the existing APIs are badly named because they conflated intent with implementation details; that is no reason to keep wilfully making the same mistake.

At least with Arm CCA, the architecture enforces *confidentiality* pretty much orthogonally to encryption - if your threat model excludes physical attacks against DRAM, you can still have Realms isolated from each other (and of course other execution states) without even implementing the memory encryption feature; conversely if you do have it, then even all the shared/host memory may still be physically encrypted, it just has its own context (key) distinct from the Realm ones. Similarly, while it's not a "true" CoCo environment, pKVM has a similar notion of shared vs. private which can benefit from piggy-backing off much of the CoCo infrastructure in places like the DMA layer, but has nothing whatsoever to do with actual encryption.

Furthermore, "shared" is just shorter and more readable, even before I invoke the previous discussion of why it should be "unencrypted" rather than "decrypted" anyway ;)

Signed-off-by: Jiri Pirko jiri@nvidia.com

v3->v4:

• added some sanity checks to dma_map_phys and dma_unmap_phys
• enhanced documentation of DMA_ATTR_CC_DECRYPTED attr

v1->v2:

• rebased on top of recent dma-mapping-fixes

---

include/linux/dma-mapping.h | 10 ++++++++++ include/trace/events/dma.h | 3 ++- kernel/dma/direct.h | 14 +++++++++++--- kernel/dma/mapping.c | 13 +++++++++++-- 4 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h index 29973baa0581..476964d2b22f 100644 --- a/include/linux/dma-mapping.h +++ b/include/linux/dma-mapping.h @@ -85,6 +85,16 @@

• a cacheline must have this attribute for this to be considered safe.

*/ #define DMA_ATTR_CPU_CACHE_CLEAN (1UL << 11) +/*

• • DMA_ATTR_CC_DECRYPTED: Indicates the DMA mapping is decrypted (shared) for
• • confidential computing guests. For normal system memory the caller must have
• • called set_memory_decrypted(), and pgprot_decrypted must be used when
• • creating CPU PTEs for the mapping. The same decrypted semantic may be passed
• • to the vIOMMU when it sets up the IOPTE. For MMIO use together with

That being "the vIOMMU" that you said doesn't exist, and which is explicitly not supported?...

• • DMA_ATTR_MMIO to indicate decrypted MMIO. Unless DMA_ATTR_MMIO is provided
• • a struct page is required.
• */

+#define DMA_ATTR_CC_DECRYPTED (1UL << 12) /*

• A dma_addr_t can hold any valid DMA or bus address for the platform. It can

diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h index 33e99e792f1a..b8082d5177c4 100644 --- a/include/trace/events/dma.h +++ b/include/trace/events/dma.h @@ -32,7 +32,8 @@ TRACE_DEFINE_ENUM(DMA_NONE); { DMA_ATTR_ALLOC_SINGLE_PAGES, "ALLOC_SINGLE_PAGES" }, \ { DMA_ATTR_NO_WARN, "NO_WARN" }, \ { DMA_ATTR_PRIVILEGED, "PRIVILEGED" }, \

• { DMA_ATTR_MMIO, "MMIO" })
  

• { DMA_ATTR_MMIO, "MMIO" }, \
  

• { DMA_ATTR_CC_DECRYPTED, "CC_DECRYPTED" })
  

DECLARE_EVENT_CLASS(dma_map, TP_PROTO(struct device *dev, phys_addr_t phys_addr, dma_addr_t dma_addr, diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h index e89f175e9c2d..c047a9d0fda3 100644 --- a/kernel/dma/direct.h +++ b/kernel/dma/direct.h @@ -84,16 +84,24 @@ static inline dma_addr_t dma_direct_map_phys(struct device *dev, dma_addr_t dma_addr; if (is_swiotlb_force_bounce(dev)) {

• if (attrs & DMA_ATTR_MMIO)
  

• 	return DMA_MAPPING_ERROR;
  

• if (!(attrs & DMA_ATTR_CC_DECRYPTED)) {
  

• 	if (attrs & DMA_ATTR_MMIO)
  

• 		return DMA_MAPPING_ERROR;
  

• return swiotlb_map(dev, phys, size, dir, attrs);
  

• 	return swiotlb_map(dev, phys, size, dir, attrs);
  

• }
  

• } else if (attrs & DMA_ATTR_CC_DECRYPTED) {

• return DMA_MAPPING_ERROR;
  

  }

if (attrs & DMA_ATTR_MMIO) { dma_addr = phys; if (unlikely(!dma_capable(dev, dma_addr, size, false))) goto err_overflow;

• } else if (attrs & DMA_ATTR_CC_DECRYPTED) {

• dma_addr = phys_to_dma_unencrypted(dev, phys);
  

• if (unlikely(!dma_capable(dev, dma_addr, size, false)))
  

• 	goto err_overflow;
  

  } else { dma_addr = phys_to_dma(dev, phys); if (unlikely(!dma_capable(dev, dma_addr, size, true)) ||

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c index 3928a509c44c..abb0c88b188b 100644 --- a/kernel/dma/mapping.c +++ b/kernel/dma/mapping.c @@ -157,6 +157,7 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size, { const struct dma_map_ops *ops = get_dma_ops(dev); bool is_mmio = attrs & DMA_ATTR_MMIO;

• bool is_cc_decrypted = attrs & DMA_ATTR_CC_DECRYPTED; dma_addr_t addr = DMA_MAPPING_ERROR;

BUG_ON(!valid_dma_direction(dir)); @@ -165,8 +166,11 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size, return DMA_MAPPING_ERROR; if (dma_map_direct(dev, ops) ||

•    (!is_mmio && arch_dma_map_phys_direct(dev, phys + size)))
  

•    (!is_mmio && !is_cc_decrypted &&
  

•     arch_dma_map_phys_direct(dev, phys + size)))
  

  addr = dma_direct_map_phys(dev, phys, size, dir, attrs);
• else if (is_cc_decrypted)

• return DMA_MAPPING_ERROR;
  

  else if (use_dma_iommu(dev))

...although, why *shouldn't* this be allowed with a vIOMMU? (Especially given that a vIOMMU for untrusted devices can be emulated by the host VMM without the CoCo hypervisor having to care at all - again, at least on Arm and other architectures where IOMMUs are regular driver model devices)

addr = iommu_dma_map_phys(dev, phys, size, dir, attrs);

else if (ops->map_phys)

Or indeed any other non-direct ops? Obviously all the legacy architectures like Alpha are never going to see this or care, but I could imagine Xen and possibly PowerPC might.

Thanks, Robin.

@@ -203,11 +207,16 @@ void dma_unmap_phys(struct device *dev, dma_addr_t addr, size_t size, { const struct dma_map_ops *ops = get_dma_ops(dev); bool is_mmio = attrs & DMA_ATTR_MMIO;

• bool is_cc_decrypted = attrs & DMA_ATTR_CC_DECRYPTED;

BUG_ON(!valid_dma_direction(dir));

• if (dma_map_direct(dev, ops) ||

•    (!is_mmio && arch_dma_unmap_phys_direct(dev, addr + size)))
  

•    (!is_mmio && !is_cc_decrypted &&
  

•     arch_dma_unmap_phys_direct(dev, addr + size)))
  

  dma_direct_unmap_phys(dev, addr, size, dir, attrs);
• else if (is_cc_decrypted)

• return;
  

  else if (use_dma_iommu(dev)) iommu_dma_unmap_phys(dev, addr, size, dir, attrs); else if (ops->unmap_phys)

On Mon, Mar 16, 2026 at 5:59 AM Jiri Pirko jiri@resnulli.us wrote:

From: Jiri Pirko jiri@nvidia.com

Add a new "system_cc_decrypted" dma-buf heap to allow userspace to allocate decrypted (shared) memory for confidential computing (CoCo) VMs.

On CoCo VMs, guest memory is encrypted by default. The hardware uses an encryption bit in page table entries (C-bit on AMD SEV, "shared" bit on Intel TDX) to control whether a given memory access is encrypted or decrypted. The kernel's direct map is set up with encryption enabled, so pages returned by alloc_pages() are encrypted in the direct map by default. To make this memory usable for devices that do not support DMA to encrypted memory (no TDISP support), it has to be explicitly decrypted. A couple of things are needed to properly handle decrypted memory for the dma-buf use case:

• set_memory_decrypted() on the direct map after allocation: Besides clearing the encryption bit in the direct map PTEs, this also notifies the hypervisor about the page state change. On free, the inverse set_memory_encrypted() must be called before returning pages to the allocator. If re-encryption fails, pages are intentionally leaked to prevent decrypted memory from being reused as private.

• pgprot_decrypted() for userspace and kernel virtual mappings: Any new mapping of the decrypted pages, be it to userspace via mmap or to kernel vmalloc space via vmap, creates PTEs independent of the direct map. These must also have the encryption bit cleared, otherwise accesses through them would see encrypted (garbage) data.

• DMA_ATTR_CC_DECRYPTED for DMA mapping: Since the pages are already decrypted, the DMA API needs to be informed via DMA_ATTR_CC_DECRYPTED so it can map them correctly as unencrypted for device access.

On non-CoCo VMs, the system_cc_decrypted heap is not registered to prevent misuse by userspace that does not understand the security implications of explicitly decrypted memory.

Signed-off-by: Jiri Pirko jiri@nvidia.com

v2->v3:

• removed couple of leftovers from headers

v1->v2:

• fixed build errors on s390 by including mem_encrypt.h
• converted system heap flag implementation to a separate heap

---

drivers/dma-buf/heaps/system_heap.c | 103 ++++++++++++++++++++++++++-- 1 file changed, 98 insertions(+), 5 deletions(-)

diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c index b3650d8fd651..a525e9aaaffa 100644 --- a/drivers/dma-buf/heaps/system_heap.c +++ b/drivers/dma-buf/heaps/system_heap.c @@ -10,17 +10,25 @@

• Andrew F. Davis <afd@ti.com>
  

*/

+#include <linux/cc_platform.h> #include <linux/dma-buf.h> #include <linux/dma-mapping.h> #include <linux/dma-heap.h> #include <linux/err.h> #include <linux/highmem.h> +#include <linux/mem_encrypt.h> #include <linux/mm.h> +#include <linux/set_memory.h> #include <linux/module.h> +#include <linux/pgtable.h> #include <linux/scatterlist.h> #include <linux/slab.h> #include <linux/vmalloc.h>

+struct system_heap_priv {

•   bool decrypted;
  

+};

Hi Jiri,

I wonder if it'd better to call this cc_decrypted (or I guess cc_shared based on Robin's comment in the previous patch) like the DMA attr? There's a separate effort for "restricted" heaps with TEE for (encrypted) video playback, which doesn't involve VMs or RDMA. I think the cc_ prefix might help avoid any confusion between the usecase here and restricted heaps.

struct system_heap_buffer { struct dma_heap *heap; struct list_head attachments; @@ -29,6 +37,7 @@ struct system_heap_buffer { struct sg_table sg_table; int vmap_cnt; void *vaddr;

•   bool decrypted;
  

};

struct dma_heap_attachment { @@ -36,6 +45,7 @@ struct dma_heap_attachment { struct sg_table table; struct list_head list; bool mapped;

•   bool decrypted;
  

};

#define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO) @@ -52,6 +62,34 @@ static gfp_t order_flags[] = {HIGH_ORDER_GFP, HIGH_ORDER_GFP, LOW_ORDER_GFP}; static const unsigned int orders[] = {8, 4, 0}; #define NUM_ORDERS ARRAY_SIZE(orders)

+static int system_heap_set_page_decrypted(struct page *page) +{

•   unsigned long addr = (unsigned long)page_address(page);
  

•   unsigned int nr_pages = 1 << compound_order(page);
  

•   int ret;
  

•   ret = set_memory_decrypted(addr, nr_pages);
  

•   if (ret)
  

•           pr_warn_ratelimited("dma-buf system heap: failed to decrypt page at %p\n",
  

•                               page_address(page));
  

•   return ret;
  

+}

+static int system_heap_set_page_encrypted(struct page *page) +{

•   unsigned long addr = (unsigned long)page_address(page);
  

•   unsigned int nr_pages = 1 << compound_order(page);
  

•   int ret;
  

•   ret = set_memory_encrypted(addr, nr_pages);
  

•   if (ret)
  

•           pr_warn_ratelimited("dma-buf system heap: failed to re-encrypt page at %p, leaking memory\n",
  

•                               page_address(page));
  

•   return ret;
  

+}

static int dup_sg_table(struct sg_table *from, struct sg_table *to) { struct scatterlist *sg, *new_sg; @@ -90,6 +128,7 @@ static int system_heap_attach(struct dma_buf *dmabuf, a->dev = attachment->dev; INIT_LIST_HEAD(&a->list); a->mapped = false;

•   a->decrypted = buffer->decrypted;
  
    attachment->priv = a;
  
  

@@ -119,9 +158,11 @@ static struct sg_table *system_heap_map_dma_buf(struct dma_buf_attachment *attac { struct dma_heap_attachment *a = attachment->priv; struct sg_table *table = &a->table;

•   unsigned long attrs;
    int ret;
  
  

•   ret = dma_map_sgtable(attachment->dev, table, direction, 0);
  

•   attrs = a->decrypted ? DMA_ATTR_CC_DECRYPTED : 0;
  

•   ret = dma_map_sgtable(attachment->dev, table, direction, attrs);
    if (ret)
            return ERR_PTR(ret);
  
  

@@ -188,8 +229,13 @@ static int system_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) unsigned long addr = vma->vm_start; unsigned long pgoff = vma->vm_pgoff; struct scatterlist *sg;

•   pgprot_t prot;
    int i, ret;
  
  

•   prot = vma->vm_page_prot;
  

•   if (buffer->decrypted)
  

•           prot = pgprot_decrypted(prot);
  

•   for_each_sgtable_sg(table, sg, i) {
            unsigned long n = sg->length >> PAGE_SHIFT;
  
  

@@ -206,8 +252,7 @@ static int system_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) if (addr + size > vma->vm_end) size = vma->vm_end - addr;

•           ret = remap_pfn_range(vma, addr, page_to_pfn(page),
  

•                           size, vma->vm_page_prot);
  

•           ret = remap_pfn_range(vma, addr, page_to_pfn(page), size, prot);
            if (ret)
                    return ret;
  
  

@@ -225,6 +270,7 @@ static void *system_heap_do_vmap(struct system_heap_buffer *buffer) struct page **pages = vmalloc(sizeof(struct page *) * npages); struct page **tmp = pages; struct sg_page_iter piter;

•   pgprot_t prot;
    void *vaddr;
  
    if (!pages)
  

@@ -235,7 +281,10 @@ static void *system_heap_do_vmap(struct system_heap_buffer *buffer) *tmp++ = sg_page_iter_page(&piter); }

•   vaddr = vmap(pages, npages, VM_MAP, PAGE_KERNEL);
  

•   prot = PAGE_KERNEL;
  

•   if (buffer->decrypted)
  

•           prot = pgprot_decrypted(prot);
  

•   vaddr = vmap(pages, npages, VM_MAP, prot);
    vfree(pages);
  
    if (!vaddr)
  

@@ -296,6 +345,14 @@ static void system_heap_dma_buf_release(struct dma_buf *dmabuf) for_each_sgtable_sg(table, sg, i) { struct page *page = sg_page(sg);

•           /*
  

•            * Intentionally leak pages that cannot be re-encrypted
  

•            * to prevent decrypted memory from being reused.
  

•            */
  

•           if (buffer->decrypted &&
  

•               system_heap_set_page_encrypted(page))
  

•                   continue;
  

•           __free_pages(page, compound_order(page));
    }
    sg_free_table(table);
  

@@ -347,6 +404,8 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap, DEFINE_DMA_BUF_EXPORT_INFO(exp_info); unsigned long size_remaining = len; unsigned int max_order = orders[0];

•   struct system_heap_priv *priv = dma_heap_get_drvdata(heap);
  

•   bool decrypted = priv->decrypted;
    struct dma_buf *dmabuf;
    struct sg_table *table;
    struct scatterlist *sg;
  

@@ -362,6 +421,7 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap, mutex_init(&buffer->lock); buffer->heap = heap; buffer->len = len;

•   buffer->decrypted = decrypted;
  
    INIT_LIST_HEAD(&pages);
    i = 0;
  

@@ -396,6 +456,14 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap, list_del(&page->lru); }

•   if (decrypted) {
  

•           for_each_sgtable_sg(table, sg, i) {
  

•                   ret = system_heap_set_page_decrypted(sg_page(sg));
  

•                   if (ret)
  

•                           goto free_pages;
  

•           }
  

•   }
  

•   /* create the dmabuf */
    exp_info.exp_name = dma_heap_get_name(heap);
    exp_info.ops = &system_heap_buf_ops;
  

@@ -413,6 +481,13 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap, for_each_sgtable_sg(table, sg, i) { struct page *p = sg_page(sg);

•           /*
  

•            * Intentionally leak pages that cannot be re-encrypted
  

•            * to prevent decrypted memory from being reused.
  

•            */
  

•           if (buffer->decrypted &&
  

•               system_heap_set_page_encrypted(p))
  

•                   continue;
            __free_pages(p, compound_order(p));
    }
    sg_free_table(table);
  

@@ -428,6 +503,14 @@ static const struct dma_heap_ops system_heap_ops = { .allocate = system_heap_allocate, };

+static struct system_heap_priv system_heap_priv = {

•   .decrypted = false,
  

+};

+static struct system_heap_priv system_heap_cc_decrypted_priv = {

•   .decrypted = true,
  

+};

static int __init system_heap_create(void) { struct dma_heap_export_info exp_info; @@ -435,8 +518,18 @@ static int __init system_heap_create(void)

    exp_info.name = "system";
    exp_info.ops = &system_heap_ops;

•   exp_info.priv = NULL;
  

•   exp_info.priv = &system_heap_priv;
  

•   sys_heap = dma_heap_add(&exp_info);
  

•   if (IS_ERR(sys_heap))
  

•           return PTR_ERR(sys_heap);
  

•   if (IS_ENABLED(CONFIG_HIGHMEM) ||
  

•       !cc_platform_has(CC_ATTR_MEM_ENCRYPT))
  

•           return 0;
  
  

•   exp_info.name = "system_cc_decrypted";
  

•   exp_info.priv = &system_heap_cc_decrypted_priv;
    sys_heap = dma_heap_add(&exp_info);
    if (IS_ERR(sys_heap))
            return PTR_ERR(sys_heap);
  

-- 2.51.1