[PATCH v11 10/16] KVM: guest_memfd: Add flag to remove from direct map

Kalyazin, Nikita
2026-03-17T14:12:34Z

From: Patrick Roy <patrick.roy@linux.dev>

Add GUEST_MEMFD_FLAG_NO_DIRECT_MAP flag for KVM_CREATE_GUEST_MEMFD()
ioctl. When set, guest_memfd folios will be removed from the direct map
after preparation, with direct map entries only restored when the folios
are freed.

To ensure these folios do not end up in places where the kernel cannot
deal with them, set AS_NO_DIRECT_MAP on the guest_memfd's struct
address_space if GUEST_MEMFD_FLAG_NO_DIRECT_MAP is requested.

Note that this flag causes removal of direct map entries for all
guest_memfd folios independent of whether they are "shared" or "private"
(although current guest_memfd only supports either all folios in the
"shared" state, or all folios in the "private" state if
GUEST_MEMFD_FLAG_MMAP is not set). The usecase for removing direct map
entries of also the shared parts of guest_memfd are a special type of
non-CoCo VM where, host userspace is trusted to have access to all of
guest memory, but where Spectre-style transient execution attacks
through the host kernel's direct map should still be mitigated. In this
setup, KVM retains access to guest memory via userspace mappings of
guest_memfd, which are reflected back into KVM's memslots via
userspace_addr. This is needed for things like MMIO emulation on x86_64
to work.

Direct map entries are zapped right before guest or userspace mappings
of gmem folios are set up, e.g. in kvm_gmem_fault_user_mapping() or
kvm_gmem_get_pfn() [called from the KVM MMU code]. The only place where
a gmem folio can be allocated without being mapped anywhere is
kvm_gmem_populate(), where handling potential failures of direct map
removal is not possible (by the time direct map removal is attempted,
the folio is already marked as prepared, meaning attempting to re-try
kvm_gmem_populate() would just result in -EEXIST without fixing up the
direct map state). These folios are then removed form the direct map
upon kvm_gmem_get_pfn(), e.g. when they are mapped into the guest later.

Signed-off-by: Patrick Roy <patrick.roy@linux.dev>
Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
---
Documentation/virt/kvm/api.rst | 21 ++++++-----
include/linux/kvm_host.h | 3 ++
include/uapi/linux/kvm.h | 1 +
virt/kvm/guest_memfd.c | 67 ++++++++++++++++++++++++++++++++--
4 files changed, 79 insertions(+), 13 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 032516783e96..8feec77b03fe 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6439,15 +6439,18 @@ a single guest_memfd file, but the bound ranges must not overlap).
The capability KVM_CAP_GUEST_MEMFD_FLAGS enumerates the `flags` that can be
specified via KVM_CREATE_GUEST_MEMFD. Currently defined flags:

- ============================ ================================================
- GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file
- descriptor.
- GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during
- KVM_CREATE_GUEST_MEMFD (memory files created
- without INIT_SHARED will be marked private).
- Shared memory can be faulted into host userspace
- page tables. Private memory cannot.
- ============================ ================================================
+ ============================== ================================================
+ GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file
+ descriptor.
+ GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during
+ KVM_CREATE_GUEST_MEMFD (memory files created
+ without INIT_SHARED will be marked private).
+ Shared memory can be faulted into host userspace
+ page tables. Private memory cannot.
+ GUEST_MEMFD_FLAG_NO_DIRECT_MAP The guest_memfd instance will unmap the memory
+ backing it from the kernel's address space
+ before passing it off to userspace or the guest.
+ ============================== ================================================

When the KVM MMU performs a PFN lookup to service a guest fault and the backing
guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ce8c5fdf2752..c95747e2278c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -738,6 +738,9 @@ static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
if (!kvm || kvm_arch_supports_gmem_init_shared(kvm))
flags |= GUEST_MEMFD_FLAG_INIT_SHARED;

+ if (!kvm || kvm_arch_gmem_supports_no_direct_map(kvm))
+ flags |= GUEST_MEMFD_FLAG_NO_DIRECT_MAP;
+
return flags;
}
#endif
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 80364d4dbebb..d864f67efdb7 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1642,6 +1642,7 @@ struct kvm_memory_attributes {
#define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd)
#define GUEST_MEMFD_FLAG_MMAP (1ULL << 0)
#define GUEST_MEMFD_FLAG_INIT_SHARED (1ULL << 1)
+#define GUEST_MEMFD_FLAG_NO_DIRECT_MAP (1ULL << 2)

struct kvm_create_guest_memfd {
__u64 size;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 651649623448..c9344647579c 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -7,6 +7,7 @@
#include <linux/mempolicy.h>
#include <linux/pseudo_fs.h>
#include <linux/pagemap.h>
+#include <linux/set_memory.h>

#include "kvm_mm.h"

@@ -76,6 +77,35 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slo
return 0;
}

+#define KVM_GMEM_FOLIO_NO_DIRECT_MAP BIT(0)
+
+static bool kvm_gmem_folio_no_direct_map(struct folio *folio)
+{
+ return ((u64)folio->private) & KVM_GMEM_FOLIO_NO_DIRECT_MAP;
+}
+
+static int kvm_gmem_folio_zap_direct_map(struct folio *folio)
+{
+ u64 gmem_flags = GMEM_I(folio_inode(folio))->flags;
+ int r = 0;
+
+ if (kvm_gmem_folio_no_direct_map(folio) || !(gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP))
+ goto out;
+
+ r = folio_zap_direct_map(folio);
+ if (!r)
+ folio->private = (void *)((u64)folio->private | KVM_GMEM_FOLIO_NO_DIRECT_MAP);
+
+out:
+ return r;
+}
+
+static void kvm_gmem_folio_restore_direct_map(struct folio *folio)
+{
+ folio_restore_direct_map(folio);
+ folio->private = (void *)((u64)folio->private & ~KVM_GMEM_FOLIO_NO_DIRECT_MAP);
+}
+
/*
* Process @folio, which contains @gfn, so that the guest can use it.
* The folio must be locked and the gfn must be contained in @slot.
@@ -388,11 +418,17 @@ static bool kvm_gmem_supports_mmap(struct inode *inode)
return GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_MMAP;
}

+static bool kvm_gmem_no_direct_map(struct inode *inode)
+{
+ return GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP;
+}
+
static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
{
struct inode *inode = file_inode(vmf->vma->vm_file);
struct folio *folio;
vm_fault_t ret = VM_FAULT_LOCKED;
+ int err;

if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
return VM_FAULT_SIGBUS;
@@ -418,6 +454,14 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
folio_mark_uptodate(folio);
}

+ if (kvm_gmem_no_direct_map(folio_inode(folio))) {
+ err = kvm_gmem_folio_zap_direct_map(folio);
+ if (err) {
+ ret = vmf_error(err);
+ goto out_folio;
+ }
+ }
+
vmf->page = folio_file_page(folio, vmf->pgoff);

out_folio:
@@ -528,6 +572,9 @@ static void kvm_gmem_free_folio(struct folio *folio)
kvm_pfn_t pfn = page_to_pfn(page);
int order = folio_order(folio);

+ if (kvm_gmem_folio_no_direct_map(folio))
+ kvm_gmem_folio_restore_direct_map(folio);
+
kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order));
}

@@ -591,6 +638,9 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
/* Unmovable mappings are supposed to be marked unevictable as well. */
WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));

+ if (flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)
+ mapping_set_no_direct_map(inode->i_mapping);
+
GMEM_I(inode)->flags = flags;

file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, &kvm_gmem_fops);
@@ -803,13 +853,22 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
}

r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
+ if (r)
+ goto out_unlock;

+ if (kvm_gmem_no_direct_map(folio_inode(folio))) {
+ r = kvm_gmem_folio_zap_direct_map(folio);
+ if (r)
+ goto out_unlock;
+ }
+
+ *page = folio_file_page(folio, index);
folio_unlock(folio);
+ return 0;

- if (!r)
- *page = folio_file_page(folio, index);
- else
- folio_put(folio);
+out_unlock:
+ folio_unlock(folio);
+ folio_put(folio);

return r;
}
--
2.50.1