[PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct map

Roy, Patrick
2025-09-24T15:22:49Z

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.

Add KVM_CAP_GUEST_MEMFD_NO_DIRECT_MAP to let userspace discover whether
guest_memfd supports GUEST_MEMFD_FLAG_NO_DIRECT_MAP. Support depends on
guest_memfd itself being supported, but also on whether linux supports
manipulatomg the direct map at page granularity at all (possible most of
the time, outliers being arm64 where its impossible if the direct map
has been setup using hugepages, as arm64 cannot break these apart due to
break-before-make semantics, and powerpc, which does not select
ARCH_HAS_SET_DIRECT_MAP, though also doesn't support guest_memfd
anyway).

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 <roypat@amazon.co.uk>
---
Documentation/virt/kvm/api.rst | 5 +++
arch/arm64/include/asm/kvm_host.h | 12 ++++++
include/linux/kvm_host.h | 6 +++
include/uapi/linux/kvm.h | 2 +
virt/kvm/guest_memfd.c | 61 ++++++++++++++++++++++++++++++-
virt/kvm/kvm_main.c | 5 +++
6 files changed, 90 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index c17a87a0a5ac..b52c14d58798 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6418,6 +6418,11 @@ When the capability KVM_CAP_GUEST_MEMFD_MMAP is supported, the 'flags' field
supports GUEST_MEMFD_FLAG_MMAP. Setting this flag on guest_memfd creation
enables mmap() and faulting of guest_memfd memory to host userspace.

+When the capability KVM_CAP_GMEM_NO_DIRECT_MAP is supported, the 'flags' field
+supports GUEST_MEMFG_FLAG_NO_DIRECT_MAP. Setting this flag makes the guest_memfd
+instance behave similarly to memfd_secret, and unmaps the memory backing it from
+the kernel's address space after allocation.
+
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
consumed from guest_memfd, regardless of whether it is a shared or a private
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 2f2394cce24e..0bfd8e5fd9de 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -19,6 +19,7 @@
#include <linux/maple_tree.h>
#include <linux/percpu.h>
#include <linux/psci.h>
+#include <linux/set_memory.h>
#include <asm/arch_gicv3.h>
#include <asm/barrier.h>
#include <asm/cpufeature.h>
@@ -1706,5 +1707,16 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt);
void get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg, u64 *res0, u64 *res1);
void check_feature_map(void);

+#ifdef CONFIG_KVM_GUEST_MEMFD
+static inline bool kvm_arch_gmem_supports_no_direct_map(void)
+{
+ /*
+ * Without FWB, direct map access is needed in kvm_pgtable_stage2_map(),
+ * as it calls dcache_clean_inval_poc().
+ */
+ return can_set_direct_map() && cpus_have_final_cap(ARM64_HAS_STAGE2_FWB);
+}
+#define kvm_arch_gmem_supports_no_direct_map kvm_arch_gmem_supports_no_direct_map
+#endif /* CONFIG_KVM_GUEST_MEMFD */

#endif /* __ARM64_KVM_HOST_H__ */
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1d0585616aa3..73a15cade54a 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -731,6 +731,12 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
bool kvm_arch_supports_gmem_mmap(struct kvm *kvm);
#endif

+#ifdef CONFIG_KVM_GUEST_MEMFD
+#ifndef kvm_arch_gmem_supports_no_direct_map
+#define kvm_arch_gmem_supports_no_direct_map can_set_direct_map
+#endif
+#endif /* CONFIG_KVM_GUEST_MEMFD */
+
#ifndef kvm_arch_has_readonly_mem
static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
{
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 6efa98a57ec1..33c8e8946019 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -963,6 +963,7 @@ struct kvm_enable_cap {
#define KVM_CAP_RISCV_MP_STATE_RESET 242
#define KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED 243
#define KVM_CAP_GUEST_MEMFD_MMAP 244
+#define KVM_CAP_GUEST_MEMFD_NO_DIRECT_MAP 245

struct kvm_irq_routing_irqchip {
__u32 irqchip;
@@ -1600,6 +1601,7 @@ 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_NO_DIRECT_MAP (1ULL << 1)

struct kvm_create_guest_memfd {
__u64 size;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 55b8d739779f..b7129c4868c5 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -4,6 +4,9 @@
#include <linux/kvm_host.h>
#include <linux/pagemap.h>
#include <linux/anon_inodes.h>
+#include <linux/set_memory.h>
+
+#include <asm/tlbflush.h>

#include "kvm_mm.h"

@@ -42,6 +45,44 @@ 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)
+{
+ if (kvm_gmem_folio_no_direct_map(folio))
+ return 0;
+
+ int r = set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio),
+ false);
+
+ if (!r) {
+ unsigned long addr = (unsigned long) folio_address(folio);
+ folio->private = (void *) ((u64) folio->private & KVM_GMEM_FOLIO_NO_DIRECT_MAP);
+ flush_tlb_kernel_range(addr, addr + folio_size(folio));
+ }
+
+ return r;
+}
+
+static void kvm_gmem_folio_restore_direct_map(struct folio *folio)
+{
+ /*
+ * Direct map restoration cannot fail, as the only error condition
+ * for direct map manipulation is failure to allocate page tables
+ * when splitting huge pages, but this split would have already
+ * happened in set_direct_map_invalid_noflush() in kvm_gmem_folio_zap_direct_map().
+ * Thus set_direct_map_valid_noflush() here only updates prot bits.
+ */
+ if (kvm_gmem_folio_no_direct_map(folio))
+ set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio),
+ true);
+}
+
static inline void kvm_gmem_mark_prepared(struct folio *folio)
{
folio_mark_uptodate(folio);
@@ -324,13 +365,14 @@ 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;

folio = kvm_gmem_get_folio(inode, vmf->pgoff);
if (IS_ERR(folio)) {
- int err = PTR_ERR(folio);
+ err = PTR_ERR(folio);

if (err == -EAGAIN)
return VM_FAULT_RETRY;
@@ -348,6 +390,13 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
kvm_gmem_mark_prepared(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:
@@ -435,6 +484,8 @@ static void kvm_gmem_free_folio(struct folio *folio)
kvm_pfn_t pfn = page_to_pfn(page);
int order = folio_order(folio);

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

@@ -499,6 +550,9 @@ 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);
+
kvm_get_kvm(kvm);
gmem->kvm = kvm;
xa_init(&gmem->bindings);
@@ -523,6 +577,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
if (kvm_arch_supports_gmem_mmap(kvm))
valid_flags |= GUEST_MEMFD_FLAG_MMAP;

+ if (kvm_arch_gmem_supports_no_direct_map())
+ valid_flags |= GUEST_MEMFD_FLAG_NO_DIRECT_MAP;
+
if (flags & ~valid_flags)
return -EINVAL;

@@ -687,6 +744,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
if (!is_prepared)
r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);

+ kvm_gmem_folio_zap_direct_map(folio);
+
folio_unlock(folio);

if (!r)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 18f29ef93543..b5e702d95230 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -65,6 +65,7 @@
#include <trace/events/kvm.h>

#include <linux/kvm_dirty_ring.h>
+#include <linux/set_memory.h>

/* Worst case buffer size needed for holding an integer. */
@@ -4916,6 +4917,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
return kvm_supported_mem_attributes(kvm);
#endif
#ifdef CONFIG_KVM_GUEST_MEMFD
+ case KVM_CAP_GUEST_MEMFD_NO_DIRECT_MAP:
+ if (!kvm_arch_gmem_supports_no_direct_map())
+ return 0;
+ fallthrough;
case KVM_CAP_GUEST_MEMFD:
return 1;
case KVM_CAP_GUEST_MEMFD_MMAP:
--
2.51.0