Message-ID: <CAEvNRgEzVhEzr-3GWTsE7GSBsPdvVLq7WFEeLHzcmMe=R9S51w@mail.gmail.com>
Date: Thu, 15 Jan 2026 12:00:30 -0800
From: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [PATCH v9 07/13] KVM: guest_memfd: Add flag to remove from direct map

"Kalyazin, Nikita" <kalyazin@...zon.co.uk> writes:

> From: Patrick Roy <patrick.roy@...ux.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@...ux.dev>
> Signed-off-by: Nikita Kalyazin <kalyazin@...zon.com>
> ---
>  Documentation/virt/kvm/api.rst | 22 ++++++++------
>  include/linux/kvm_host.h       | 12 ++++++++
>  include/uapi/linux/kvm.h       |  1 +
>  virt/kvm/guest_memfd.c         | 54 ++++++++++++++++++++++++++++++++++
>  4 files changed, 80 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 01a3abef8abb..c5f54f1370c8 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6440,15 +6440,19 @@ 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 behave similarly
> +                                 to memfd_secret, and unmaps the memory backing

Perhaps the reference to memfd_secret can be dropped to avoid anyone
assuming further similarities between guest_memfd and memfd_secret. This
could just say that "The guest_memfd instance will unmap the memory
backing it from the kernel's address space...".

> +                                 it from the kernel's address space before
> +                                 being passed 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 27796a09d29b..d4d5306075bf 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -738,10 +738,22 @@ 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_arch_gmem_supports_no_direct_map())
> +		flags |= GUEST_MEMFD_FLAG_NO_DIRECT_MAP;
> +
>  	return flags;
>  }
>  #endif
>
> +#ifdef CONFIG_KVM_GUEST_MEMFD
> +#ifndef kvm_arch_gmem_supports_no_direct_map
> +static inline bool kvm_arch_gmem_supports_no_direct_map(void)
> +{
> +	return false;
> +}
> +#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 dddb781b0507..60341e1ba1be 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1612,6 +1612,7 @@ 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 92e7f8c1f303..43f64c11467a 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -7,6 +7,9 @@
>  #include <linux/mempolicy.h>
>  #include <linux/pseudo_fs.h>
>  #include <linux/pagemap.h>
> +#include <linux/set_memory.h>
> +
> +#include <asm/tlbflush.h>
>
>  #include "kvm_mm.h"
>
> @@ -76,6 +79,43 @@ 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;

Nit: I think there shouldn't be a space between (u64) and what's being casted.

> +}
> +
> +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;
> +
> +	folio->private = (void *)((u64)folio->private | KVM_GMEM_FOLIO_NO_DIRECT_MAP);
> +	r = folio_zap_direct_map(folio);
> +
> +out:
> +	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 folio_zap_direct_map() in kvm_gmem_folio_zap_direct_map().
> +	 * Thus folio_restore_direct_map() here only updates prot bits.
> +	 */

Thanks for this comment :)

> +	if (kvm_gmem_folio_no_direct_map(folio)) {
> +		WARN_ON_ONCE(folio_restore_direct_map(folio));
> +		folio->private = (void *)((u64)folio->private & ~KVM_GMEM_FOLIO_NO_DIRECT_MAP);
> +	}
> +}
> +
>  static inline void kvm_gmem_mark_prepared(struct folio *folio)
>  {
>  	folio_mark_uptodate(folio);
> @@ -398,6 +438,7 @@ 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;
> @@ -423,6 +464,12 @@ 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);

Perhaps the check for gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP should
be done here before making the call to kvm_gmem_folio_zap_direct_map()
to make it more obvious that zapping is conditional.

Perhaps also add a check for kvm_arch_gmem_supports_no_direct_map() so
this call can be completely removed by the compiler if it wasn't
compiled in.

The kvm_gmem_folio_no_direct_map() check should probably remain in
kvm_gmem_folio_zap_direct_map() since that's a "if already zapped, don't
zap again" check.

> +	if (err) {
> +		ret = vmf_error(err);
> +		goto out_folio;
> +	}
> +
>  	vmf->page = folio_file_page(folio, vmf->pgoff);
>
>  out_folio:
> @@ -533,6 +580,8 @@ 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);
> +

I can't decide if the kvm_gmem_folio_no_direct_map(folio) should be in
the caller or within kvm_gmem_folio_restore_direct_map(), since this
time it's a folio-specific property being checked.

Perhaps also add a check for kvm_arch_gmem_supports_no_direct_map() so
this call can be completely removed by the compiler if it wasn't
compiled in. IIUC whether the check is added in the caller or within
kvm_gmem_folio_restore_direct_map() the call can still be elided.

>  	kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order));
>  }
>
> @@ -596,6 +645,9 @@ 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);
> @@ -807,6 +859,8 @@ 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);
> +

Is there a reason why errors are not handled when faulting private memory?

>  	folio_unlock(folio);
>
>  	if (!r)
> --
> 2.50.1

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