[PATCH v3 24/25] KVM: TDX: Guard VM state transitions with "all" the locks
From: Sean Christopherson <seanjc@google.com>
Date: 2025-10-17 00:33:30
Also in:
kvm, kvm-riscv, kvmarm, linux-arm-kernel, linux-coco, linux-mips, linux-riscv, lkml, loongarch
Subsystem:
kernel virtual machine for x86 (kvm/x86), the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Sean Christopherson, Paolo Bonzini, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
Acquire kvm->lock, kvm->slots_lock, and all vcpu->mutex locks when
servicing ioctls that (a) transition the TD to a new state, i.e. when
doing INIT or FINALIZE or (b) are only valid if the TD is in a specific
state, i.e. when initializing a vCPU or memory region. Acquiring "all"
the locks fixes several KVM_BUG_ON() situations where a SEAMCALL can fail
due to racing actions, e.g. if tdh_vp_create() contends with either
tdh_mr_extend() or tdh_mr_finalize().
For all intents and purposes, the paths in question are fully serialized,
i.e. there's no reason to try and allow anything remotely interesting to
happen. Smack 'em with a big hammer instead of trying to be "nice".
Acquire kvm->lock to prevent VM-wide things from happening, slots_lock to
prevent kvm_mmu_zap_all_fast(), and _all_ vCPU mutexes to prevent vCPUs
from interefering. Use the recently-renamed kvm_arch_vcpu_unlocked_ioctl()
to service the vCPU-scoped ioctls to avoid a lock inversion problem, e.g.
due to taking vcpu->mutex outside kvm->lock.
See also commit ecf371f8b02d ("KVM: SVM: Reject SEV{-ES} intra host
migration if vCPU creation is in-flight"), which fixed a similar bug with
SEV intra-host migration where an in-flight vCPU creation could race with
a VM-wide state transition.
Define a fancy new CLASS to handle the lock+check => unlock logic with
guard()-like syntax:
CLASS(tdx_vm_state_guard, guard)(kvm);
if (IS_ERR(guard))
return PTR_ERR(guard);
to simplify juggling the many locks.
Note! Take kvm->slots_lock *after* all vcpu->mutex locks, as per KVM's
soon-to-be-documented lock ordering rules[1].
Link: https://lore.kernel.org/all/20251016235538.171962-1-seanjc@google.com (local) [1]
Reported-by: Yan Zhao <redacted>
Closes: https://lore.kernel.org/all/aLFiPq1smdzN3Ary@yzhao56-desk.sh.intel.com (local)
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/tdx.c | 63 +++++++++++++++++++++++++++++++++++-------
1 file changed, 53 insertions(+), 10 deletions(-)
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 84b5fe654c99..d6541b08423f 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c@@ -2632,6 +2632,46 @@ static int tdx_read_cpuid(struct kvm_vcpu *vcpu, u32 leaf, u32 sub_leaf, return -EIO; } +typedef void *tdx_vm_state_guard_t; + +static tdx_vm_state_guard_t tdx_acquire_vm_state_locks(struct kvm *kvm) +{ + int r; + + mutex_lock(&kvm->lock); + + if (kvm->created_vcpus != atomic_read(&kvm->online_vcpus)) { + r = -EBUSY; + goto out_err; + } + + r = kvm_lock_all_vcpus(kvm); + if (r) + goto out_err; + + /* + * Note the unintuitive ordering! vcpu->mutex must be taken outside + * kvm->slots_lock! + */ + mutex_lock(&kvm->slots_lock); + return kvm; + +out_err: + mutex_unlock(&kvm->lock); + return ERR_PTR(r); +} + +static void tdx_release_vm_state_locks(struct kvm *kvm) +{ + mutex_unlock(&kvm->slots_lock); + kvm_unlock_all_vcpus(kvm); + mutex_unlock(&kvm->lock); +} + +DEFINE_CLASS(tdx_vm_state_guard, tdx_vm_state_guard_t, + if (!IS_ERR(_T)) tdx_release_vm_state_locks(_T), + tdx_acquire_vm_state_locks(kvm), struct kvm *kvm); + static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd) { struct kvm_tdx_init_vm __user *user_data = u64_to_user_ptr(cmd->data);
@@ -2644,6 +2684,10 @@ static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd) BUILD_BUG_ON(sizeof(*init_vm) != 256 + sizeof_field(struct kvm_tdx_init_vm, cpuid)); BUILD_BUG_ON(sizeof(struct td_params) != 1024); + CLASS(tdx_vm_state_guard, guard)(kvm); + if (IS_ERR(guard)) + return PTR_ERR(guard); + if (kvm_tdx->state != TD_STATE_UNINITIALIZED) return -EINVAL;
@@ -2743,7 +2787,9 @@ static int tdx_td_finalize(struct kvm *kvm, struct kvm_tdx_cmd *cmd) { struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm); - guard(mutex)(&kvm->slots_lock); + CLASS(tdx_vm_state_guard, guard)(kvm); + if (IS_ERR(guard)) + return PTR_ERR(guard); if (!is_hkid_assigned(kvm_tdx) || kvm_tdx->state == TD_STATE_RUNNABLE) return -EINVAL;
@@ -2781,8 +2827,6 @@ int tdx_vm_ioctl(struct kvm *kvm, void __user *argp) if (r) return r; - guard(mutex)(&kvm->lock); - switch (tdx_cmd.id) { case KVM_TDX_CAPABILITIES: r = tdx_get_capabilities(&tdx_cmd);
@@ -3090,8 +3134,6 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c if (tdx->state != VCPU_TD_STATE_INITIALIZED) return -EINVAL; - guard(mutex)(&kvm->slots_lock); - /* Once TD is finalized, the initial guest memory is fixed. */ if (kvm_tdx->state == TD_STATE_RUNNABLE) return -EINVAL;
@@ -3147,7 +3189,8 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c int tdx_vcpu_unlocked_ioctl(struct kvm_vcpu *vcpu, void __user *argp) { - struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm); + struct kvm *kvm = vcpu->kvm; + struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm); struct kvm_tdx_cmd cmd; int r;
@@ -3155,12 +3198,13 @@ int tdx_vcpu_unlocked_ioctl(struct kvm_vcpu *vcpu, void __user *argp) if (r) return r; + CLASS(tdx_vm_state_guard, guard)(kvm); + if (IS_ERR(guard)) + return PTR_ERR(guard); + if (!is_hkid_assigned(kvm_tdx) || kvm_tdx->state == TD_STATE_RUNNABLE) return -EINVAL; - if (mutex_lock_killable(&vcpu->mutex)) - return -EINTR; - vcpu_load(vcpu); switch (cmd.id) {
@@ -3177,7 +3221,6 @@ int tdx_vcpu_unlocked_ioctl(struct kvm_vcpu *vcpu, void __user *argp) vcpu_put(vcpu); - mutex_unlock(&vcpu->mutex); return r; }
--
2.51.0.858.gf9c4a03a3a-goog