KVM: arm64: vgic-init: Plug vCPU vs. VGIC creation race

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 <glider@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Tested-by: Alexander Potapenko <glider@google.com>
Link: https://lore.kernel.org/r/20250523194722.4066715-6-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>