From fe2bf6234e947bf5544db6d386af1df2a8db80f3 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 3 Oct 2025 16:25:55 -0700 Subject: KVM: guest_memfd: Add INIT_SHARED flag, reject user page faults if not set Add a guest_memfd flag to allow userspace to state that the underlying memory should be configured to be initialized as shared, and reject user page faults if the guest_memfd instance's memory isn't shared. Because KVM doesn't yet support in-place private<=>shared conversions, all guest_memfd memory effectively follows the initial state. Alternatively, KVM could deduce the initial state based on MMAP, which for all intents and purposes is what KVM currently does. However, implicitly deriving the default state based on MMAP will result in a messy ABI when support for in-place conversions is added. For x86 CoCo VMs, which don't yet support MMAP, memory is currently private by default (otherwise the memory would be unusable). If MMAP implies memory is shared by default, then the default state for CoCo VMs will vary based on MMAP, and from userspace's perspective, will change when in-place conversion support is added. I.e. to maintain guest<=>host ABI, userspace would need to immediately convert all memory from shared=>private, which is both ugly and inefficient. The inefficiency could be avoided by adding a flag to state that memory is _private_ by default, irrespective of MMAP, but that would lead to an equally messy and hard to document ABI. Bite the bullet and immediately add a flag to control the default state so that the effective behavior is explicit and straightforward. Fixes: 3d3a04fad25a ("KVM: Allow and advertise support for host mmap() on guest_memfd files") Cc: David Hildenbrand Reviewed-by: Fuad Tabba Tested-by: Fuad Tabba Reviewed-by: Ackerley Tng Tested-by: Ackerley Tng Reviewed-by: David Hildenbrand Link: https://lore.kernel.org/r/20251003232606.4070510-3-seanjc@google.com Signed-off-by: Sean Christopherson --- virt/kvm/kvm_main.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'virt/kvm/kvm_main.c') diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index e3a268757621..5f644ca54af3 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -4930,7 +4930,8 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg) return 1; case KVM_CAP_GUEST_MEMFD_FLAGS: if (!kvm || kvm_arch_supports_gmem_mmap(kvm)) - return GUEST_MEMFD_FLAG_MMAP; + return GUEST_MEMFD_FLAG_MMAP | + GUEST_MEMFD_FLAG_INIT_SHARED; return 0; #endif -- cgit v1.2.3