From: Oliver Upton Date: Fri, 23 May 2025 19:47:22 +0000 (-0700) Subject: KVM: arm64: vgic-init: Plug vCPU vs. VGIC creation race X-Git-Tag: v6.16-rc1~78^2^2~1 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=07212d16adc7a02810e1641c2721762751ce4f88;p=linux-block.git KVM: arm64: vgic-init: Plug vCPU vs. VGIC creation race syzkaller has found another ugly race in the VGIC, this time dealing with VGIC creation. Since kvm_vgic_create() doesn't sufficiently protect against in-flight vCPU creations, it is possible to get a vCPU into the kernel w/ an in-kernel VGIC but no allocation of private IRQs: Unable to handle kernel NULL pointer dereference at virtual address 0000000000000d20 Mem abort info: ESR = 0x0000000096000046 EC = 0x25: DABT (current EL), IL = 32 bits SET = 0, FnV = 0 EA = 0, S1PTW = 0 FSC = 0x06: level 2 translation fault Data abort info: ISV = 0, ISS = 0x00000046, ISS2 = 0x00000000 CM = 0, WnR = 1, TnD = 0, TagAccess = 0 GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0 user pgtable: 4k pages, 48-bit VAs, pgdp=0000000103e4f000 [0000000000000d20] pgd=0800000102e1c403, p4d=0800000102e1c403, pud=0800000101146403, pmd=0000000000000000 Internal error: Oops: 0000000096000046 [#1] PREEMPT SMP CPU: 9 UID: 0 PID: 246 Comm: test Not tainted 6.14.0-rc6-00097-g0c90821f5db8 #16 Hardware name: linux,dummy-virt (DT) pstate: 814020c5 (Nzcv daIF +PAN -UAO -TCO +DIT -SSBS BTYPE=--) pc : _raw_spin_lock_irqsave+0x34/0x8c lr : kvm_vgic_set_owner+0x54/0xa4 sp : ffff80008086ba20 x29: ffff80008086ba20 x28: ffff0000c19b5640 x27: 0000000000000000 x26: 0000000000000000 x25: ffff0000c4879bd0 x24: 000000000000001e x23: 0000000000000000 x22: 0000000000000000 x21: ffff0000c487af80 x20: ffff0000c487af18 x19: 0000000000000000 x18: 0000001afadd5a8b x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000001 x14: ffff0000c19b56c0 x13: 0030c9adf9d9889e x12: ffffc263710e1908 x11: 0000001afb0d74f2 x10: e0966b840b373664 x9 : ec806bf7d6a57cd5 x8 : ffff80008086b980 x7 : 0000000000000001 x6 : 0000000000000001 x5 : 0000000080800054 x4 : 4ec4ec4ec4ec4ec5 x3 : 0000000000000000 x2 : 0000000000000001 x1 : 0000000000000000 x0 : 0000000000000d20 Call trace: _raw_spin_lock_irqsave+0x34/0x8c (P) kvm_vgic_set_owner+0x54/0xa4 kvm_timer_enable+0xf4/0x274 kvm_arch_vcpu_run_pid_change+0xe0/0x380 kvm_vcpu_ioctl+0x93c/0x9e0 __arm64_sys_ioctl+0xb4/0xec invoke_syscall+0x48/0x110 el0_svc_common.constprop.0+0x40/0xe0 do_el0_svc+0x1c/0x28 el0_svc+0x30/0xd0 el0t_64_sync_handler+0x10c/0x138 el0t_64_sync+0x198/0x19c Code: b9000841 d503201f 52800001 52800022 (88e17c02) ---[ end trace 0000000000000000 ]--- Plug the race by explicitly checking for an in-progress vCPU creation and failing kvm_vgic_create() when that's the case. Add some comments to document all the things kvm_vgic_create() is trying to guard against too. Reported-by: Alexander Potapenko Signed-off-by: Oliver Upton Tested-by: Alexander Potapenko Link: https://lore.kernel.org/r/20250523194722.4066715-6-oliver.upton@linux.dev Signed-off-by: Marc Zyngier --- diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c index 1f33e71c2a73..0611a1c2ce8e 100644 --- a/arch/arm64/kvm/vgic/vgic-init.c +++ b/arch/arm64/kvm/vgic/vgic-init.c @@ -84,15 +84,40 @@ int kvm_vgic_create(struct kvm *kvm, u32 type) !kvm_vgic_global_state.can_emulate_gicv2) return -ENODEV; - /* Must be held to avoid race with vCPU creation */ + /* + * Ensure mutual exclusion with vCPU creation and any vCPU ioctls by: + * + * - Holding kvm->lock to prevent KVM_CREATE_VCPU from reaching + * kvm_arch_vcpu_precreate() and ensuring created_vcpus is stable. + * This alone is insufficient, as kvm_vm_ioctl_create_vcpu() drops + * the kvm->lock before completing the vCPU creation. + */ lockdep_assert_held(&kvm->lock); + /* + * - Acquiring the vCPU mutex for every *online* vCPU to prevent + * concurrent vCPU ioctls for vCPUs already visible to userspace. + */ ret = -EBUSY; if (!lock_all_vcpus(kvm)) return ret; + /* + * - Taking the config_lock which protects VGIC data structures such + * as the per-vCPU arrays of private IRQs (SGIs, PPIs). + */ mutex_lock(&kvm->arch.config_lock); + /* + * - Bailing on the entire thing if a vCPU is in the middle of creation, + * dropped the kvm->lock, but hasn't reached kvm_arch_vcpu_create(). + * + * The whole combination of this guarantees that no vCPU can get into + * KVM with a VGIC configuration inconsistent with the VM's VGIC. + */ + if (kvm->created_vcpus != atomic_read(&kvm->online_vcpus)) + goto out_unlock; + if (irqchip_in_kernel(kvm)) { ret = -EEXIST; goto out_unlock;