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

David Hildenbrand
2025-09-25T11:00:54Z

On 24.09.25 17:22, Roy, Patrick wrote:
> 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.
> +

Do we want to document what the implication of that is? Meaning,
limitations etc. I recall that we would need the user mapping for gmem
slots to be properly set up.

Is that still the case in this patch set?

> 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 */
>

I strongly assume that the aarch64 support should be moved to a separate
patch -- if possible, see below.

> #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

Hm, wouldn't it be better to have an opt-in per arch, and really only
unlock the ones we know work (tested etc), explicitly in separate patches.

[...]

>
> #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);

empty line missing.

> + 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);
> +

I'd drop this empty line here.

> + 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>

Likely not required here.

--
Cheers

David / dhildenb