From e563592224e02f87048edee3ce3f0da16cceee88 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Thu, 22 Feb 2024 11:06:08 -0800 Subject: [PATCH] KVM: Make KVM_MEM_GUEST_MEMFD mutually exclusive with KVM_MEM_READONLY Disallow creating read-only memslots that support GUEST_MEMFD, as GUEST_MEMFD is fundamentally incompatible with KVM's semantics for read-only memslots. Read-only memslots allow the userspace VMM to emulate option ROMs by filling the backing memory with readable, executable code and data, while triggering emulated MMIO on writes. GUEST_MEMFD doesn't currently support writes from userspace and KVM doesn't support emulated MMIO on private accesses, i.e. the guest can only ever read zeros, and writes will always be treated as errors. Cc: Fuad Tabba Cc: Michael Roth Cc: Isaku Yamahata Cc: Yu Zhang Cc: Chao Peng Fixes: a7800aa80ea4 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory") Link: https://lore.kernel.org/r/20240222190612.2942589-2-seanjc@google.com Signed-off-by: Sean Christopherson --- virt/kvm/kvm_main.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 10bfc88a69f7..0f50960b0e3a 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1615,7 +1615,13 @@ static int check_memory_region_flags(struct kvm *kvm, valid_flags &= ~KVM_MEM_LOG_DIRTY_PAGES; #ifdef __KVM_HAVE_READONLY_MEM - valid_flags |= KVM_MEM_READONLY; + /* + * GUEST_MEMFD is incompatible with read-only memslots, as writes to + * read-only memslots have emulated MMIO, not page fault, semantics, + * and KVM doesn't allow emulated MMIO for private memory. + */ + if (!(mem->flags & KVM_MEM_GUEST_MEMFD)) + valid_flags |= KVM_MEM_READONLY; #endif if (mem->flags & ~valid_flags)