When accessing the ESB page of a source interrupt, the fault handler
will retrieve the page address from the XIVE interrupt 'xive_irq_data'
structure. If the associated KVM XIVE interrupt is not valid, that is
not allocated at the HW level for some reason, the fault handler will
dereference a NULL pointer leading to the oops below :

  WARNING: CPU: 40 PID: 59101 at arch/powerpc/kvm/book3s_xive_native.c:259 xive_native_esb_fault+0xe4/0x240 [kvm]
  CPU: 40 PID: 59101 Comm: qemu-system-ppc Kdump: loaded Tainted: G        W        --------- -  - 4.18.0-240.el8.ppc64le #1
  NIP:  c00800000e949fac LR: c00000000044b164 CTR: c00800000e949ec8
  REGS: c000001f69617840 TRAP: 0700   Tainted: G        W        --------- -  -  (4.18.0-240.el8.ppc64le)
  MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44044282  XER: 00000000
  CFAR: c00000000044b160 IRQMASK: 0
  GPR00: c00000000044b164 c000001f69617ac0 c00800000e96e000 c000001f69617c10
  GPR04: 05faa2b21e000080 0000000000000000 0000000000000005 ffffffffffffffff
  GPR08: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
  GPR12: c00800000e949ec8 c000001ffffd3400 0000000000000000 0000000000000000
  GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  GPR20: 0000000000000000 0000000000000000 c000001f5c065160 c000000001c76f90
  GPR24: c000001f06f20000 c000001f5c065100 0000000000000008 c000001f0eb98c78
  GPR28: c000001dcab40000 c000001dcab403d8 c000001f69617c10 0000000000000011
  NIP [c00800000e949fac] xive_native_esb_fault+0xe4/0x240 [kvm]
  LR [c00000000044b164] __do_fault+0x64/0x220
  Call Trace:
  [c000001f69617ac0] [0000000137a5dc20] 0x137a5dc20 (unreliable)
  [c000001f69617b50] [c00000000044b164] __do_fault+0x64/0x220
  [c000001f69617b90] [c000000000453838] do_fault+0x218/0x930
  [c000001f69617bf0] [c000000000456f50] __handle_mm_fault+0x350/0xdf0
  [c000001f69617cd0] [c000000000457b1c] handle_mm_fault+0x12c/0x310
  [c000001f69617d10] [c00000000007ef44] __do_page_fault+0x264/0xbb0
  [c000001f69617df0] [c00000000007f8c8] do_page_fault+0x38/0xd0
  [c000001f69617e30] [c00000000000a714] handle_page_fault+0x18/0x38
  Instruction dump:
  40c2fff0 7c2004ac 2fa90000 409e0118 73e90001 41820080 e8bd0008 7c2004ac
  7ca90074 39400000 915c0000 7929d182 <0b090000> 2fa50000 419e0080 e89e0018
  ---[ end trace 66c6ff034c53f64f ]---
  xive-kvm: xive_native_esb_fault: accessing invalid ESB page for source 8 !

Fix that by checking the validity of the KVM XIVE interrupt structure.

Fixes: 6520ca64 ("KVM: PPC: Book3S HV: XIVE: Add a mapping for the source ESB pages")
Cc: stable@vger.kernel.org # v5.2+
Reported-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201105134713.656160-1-clg@kaod.org