| Message ID | 20240809190319.1710470-2-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86: Fix multiple #PF RO infinite loop bugs | expand |
On 8/9/24 21:02, Sean Christopherson wrote: > Disallow read-only memory for all VMs with protected state, i.e. for > upcoming TDX VMs as well as ES/SNP VMs. For TDX, it's actually possible > to support read-only memory, as TDX uses EPT Violation #VE to reflect the > fault into the guest, e.g. KVM could configure read-only SPTEs with RX > protections and SUPPRESS_VE=0. But there is no strong use case for > supporting read-only memslots on TDX, e.g. the main historical usage is > to emulate option ROMs, but TDX disallows executing from shared memory. > And if someone comes along with a legitimate, strong use case, the > restriction can always be lifted for TDX. > > Don't bother trying to retroactively apply the restriction to SEV-ES > VMs that are created as type KVM_X86_DEFAULT_VM. Read-only memslots can't > possibly work for SEV-ES, i.e. disallowing such memslots is really just > means reporting an error to userspace instead of silently hanging vCPUs. > Trying to deal with the ordering between KVM_SEV_INIT and memslot creation > isn't worth the marginal benefit it would provide userspace. Queuing this one for 6.11, thanks. Paolo
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 950a03e0181e..37c4a573e5fb 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -2191,6 +2191,8 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level, #define kvm_arch_has_private_mem(kvm) false #endif +#define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state) + static inline u16 kvm_read_ldt(void) { u16 ldt; diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 689e8be873a7..62a3d1c0cc07 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -715,6 +715,13 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm) } #endif +#ifndef kvm_arch_has_readonly_mem +static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm) +{ + return IS_ENABLED(CONFIG_HAVE_KVM_READONLY_MEM); +} +#endif + struct kvm_memslots { u64 generation; atomic_long_t last_used_slot; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index d0788d0a72cc..fad2d5932844 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1578,15 +1578,14 @@ static int check_memory_region_flags(struct kvm *kvm, if (mem->flags & KVM_MEM_GUEST_MEMFD) valid_flags &= ~KVM_MEM_LOG_DIRTY_PAGES; -#ifdef CONFIG_HAVE_KVM_READONLY_MEM /* * 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)) + if (kvm_arch_has_readonly_mem(kvm) && + !(mem->flags & KVM_MEM_GUEST_MEMFD)) valid_flags |= KVM_MEM_READONLY; -#endif if (mem->flags & ~valid_flags) return -EINVAL;
Disallow read-only memslots for SEV-{ES,SNP} VM types, as KVM can't directly emulate instructions for ES/SNP, and instead the guest must explicitly request emulation. Unless the guest explicitly requests emulation without accessing memory, ES/SNP relies on KVM creating an MMIO SPTE, with the subsequent #NPF being reflected into the guest as a #VC. But for read-only memslots, KVM deliberately doesn't create MMIO SPTEs, because except for ES/SNP, doing so requires setting reserved bits in the SPTE, i.e. the SPTE can't be readable while also generating a #VC on writes. Because KVM never creates MMIO SPTEs and jumps directly to emulation, the guest never gets a #VC. And since KVM simply resumes the guest if ES/SNP guests trigger emulation, KVM effectively puts the vCPU into an infinite #NPF loop if the vCPU attempts to write read-only memory. Disallow read-only memory for all VMs with protected state, i.e. for upcoming TDX VMs as well as ES/SNP VMs. For TDX, it's actually possible to support read-only memory, as TDX uses EPT Violation #VE to reflect the fault into the guest, e.g. KVM could configure read-only SPTEs with RX protections and SUPPRESS_VE=0. But there is no strong use case for supporting read-only memslots on TDX, e.g. the main historical usage is to emulate option ROMs, but TDX disallows executing from shared memory. And if someone comes along with a legitimate, strong use case, the restriction can always be lifted for TDX. Don't bother trying to retroactively apply the restriction to SEV-ES VMs that are created as type KVM_X86_DEFAULT_VM. Read-only memslots can't possibly work for SEV-ES, i.e. disallowing such memslots is really just means reporting an error to userspace instead of silently hanging vCPUs. Trying to deal with the ordering between KVM_SEV_INIT and memslot creation isn't worth the marginal benefit it would provide userspace. Fixes: 26c44aa9e076 ("KVM: SEV: define VM types for SEV and SEV-ES") Fixes: 1dfe571c12cf ("KVM: SEV: Add initial SEV-SNP support") Cc: Peter Gonda <pgonda@google.com> Cc: Michael Roth <michael.roth@amd.com> Cc: Vishal Annapurve <vannapurve@google.com> Cc: Ackerly Tng <ackerleytng@google.com> Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/include/asm/kvm_host.h | 2 ++ include/linux/kvm_host.h | 7 +++++++ virt/kvm/kvm_main.c | 5 ++--- 3 files changed, 11 insertions(+), 3 deletions(-)