kvm->lock must be taken outside of the vcpu->mutex. Of course, the locking documentation for KVM makes this abundantly clear. Nonetheless, the locking order in KVM/arm64 has been wrong for quite a while; we acquire the kvm->lock while holding the vcpu->mutex all over the shop.

All was seemingly fine until commit 42a90008f890 ("KVM: Ensure lockdep knows about kvm->lock vs. vcpu->mutex ordering rule") caught us with our pants down, leading to lockdep barfing:

====================================================== WARNING: possible circular locking dependency detected 6.2.0-rc7+ #19 Not tainted ------------------------------------------------------ qemu-system-aar/859 is trying to acquire lock: ffff5aa69269eba0 (&host_kvm->lock){+.+.}-{3:3}, at: kvm_reset_vcpu+0x34/0x274

but task is already holding lock: ffff5aa68768c0b8 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x8c/0xba0

which lock already depends on the new lock.

Add a dedicated lock to serialize writes to VM-scoped configuration from the context of a vCPU. Protect the register width flags with the new lock, thus avoiding the need to grab the kvm->lock while holding vcpu->mutex in kvm_reset_vcpu().

Cc: stable@vger.kernel.org Reported-by: Jeremy Linton jeremy.linton@arm.com Link: https://lore.kernel.org/kvmarm/f6452cdd-65ff-34b8-bab0-5c06416da5f6@arm.com/ Tested-by: Jeremy Linton jeremy.linton@arm.com Signed-off-by: Oliver Upton oliver.upton@linux.dev --- arch/arm64/include/asm/kvm_host.h | 3 +++ arch/arm64/kvm/arm.c | 18 ++++++++++++++++++ arch/arm64/kvm/reset.c | 6 +++--- 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h index 917586237a4d..cd1ef8716719 100644 --- a/arch/arm64/include/asm/kvm_host.h +++ b/arch/arm64/include/asm/kvm_host.h @@ -199,6 +199,9 @@ struct kvm_arch { /* Mandated version of PSCI */ u32 psci_version;

+ /* Protects VM-scoped configuration data */ + struct mutex config_lock; + /* * If we encounter a data abort without valid instruction syndrome * information, report this to user space. User space can (and diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c index 647798da8c41..1620ec3d95ef 100644 --- a/arch/arm64/kvm/arm.c +++ b/arch/arm64/kvm/arm.c @@ -128,6 +128,16 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) { int ret;

+ mutex_init(&kvm->arch.config_lock); + +#ifdef CONFIG_LOCKDEP + /* Clue in lockdep that the config_lock must be taken inside kvm->lock */ + mutex_lock(&kvm->lock); + mutex_lock(&kvm->arch.config_lock); + mutex_unlock(&kvm->arch.config_lock); + mutex_unlock(&kvm->lock); +#endif + ret = kvm_share_hyp(kvm, kvm + 1); if (ret) return ret; @@ -328,6 +338,14 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)

spin_lock_init(&vcpu->arch.mp_state_lock);

+#ifdef CONFIG_LOCKDEP + /* Inform lockdep that the config_lock is acquired after vcpu->mutex */ + mutex_lock(&vcpu->mutex); + mutex_lock(&vcpu->kvm->arch.config_lock); + mutex_unlock(&vcpu->kvm->arch.config_lock); + mutex_unlock(&vcpu->mutex); +#endif + /* Force users to call KVM_ARM_VCPU_INIT */ vcpu->arch.target = -1; bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES); diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c index 9e023546bde0..b5dee8e57e77 100644 --- a/arch/arm64/kvm/reset.c +++ b/arch/arm64/kvm/reset.c @@ -205,7 +205,7 @@ static int kvm_set_vm_width(struct kvm_vcpu *vcpu)

is32bit = vcpu_has_feature(vcpu, KVM_ARM_VCPU_EL1_32BIT);

- lockdep_assert_held(&kvm->lock); + lockdep_assert_held(&kvm->arch.config_lock);

if (test_bit(KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED, &kvm->arch.flags)) { /* @@ -262,9 +262,9 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu) bool loaded; u32 pstate;

- mutex_lock(&vcpu->kvm->lock); + mutex_lock(&vcpu->kvm->arch.config_lock); ret = kvm_set_vm_width(vcpu); - mutex_unlock(&vcpu->kvm->lock); + mutex_unlock(&vcpu->kvm->arch.config_lock);

if (ret) return ret;