[PATCH] i915: Fix NULL pointer dereference in intel_dmc_update_dc6_allowed_count()

[Tao Liu <ltao@redhat.com>]

There is a NULL pointer dereference issue noticed in i915 when 2nd kernel
bootup during kdump. This will panic 2nd kernel and lead to no vmcore
generation. The issue is observed in Meteorlake CPU(cpuid: 0xA06A2):

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 0 P4D 0
    Oops: Oops: 0000 [#1] SMP NOPTI
    ...
    RIP: 0010:intel_dmc_update_dc6_allowed_count+0x16/0xa0 [i915]
    ...

It is easy to locate the NULL pointer dereference by disassembly:

    00000000001171e0 <intel_dmc_update_dc6_allowed_count>:
      1171e0:       f3 0f 1e fa             endbr64
      1171e4:       e8 00 00 00 00          call   1171e9
      1171e9:       41 55                   push   %r13
      1171eb:       41 54                   push   %r12
      1171ed:       55                      push   %rbp
      1171ee:       53                      push   %rbx
      1171ef:       4c 8b a7 18 03 00 00    mov    0x318(%rdi),%r12
      1171f6:       49 8b 2c 24             mov    (%r12),%rbp

To fix this, add a NULL pointer check before dereferencing.

Signed-off-by: Tao Liu <ltao@redhat.com>
---

The issue doesn't happen in 1st kernel, but in 2nd kernel of kdump. I'm not
an expert to i915 and unsure what lead to the NULL pointer. To help further
analysis, here is the full stack:

[    8.608520]  <TASK> 
[    8.610652]  gen9_set_dc_state.part.0+0x25d/0x2f0 [i915] 
[    8.616096]  icl_display_core_init+0x2d/0x620 [i915] 
[    8.621266]  intel_power_domains_init_hw+0x1b2/0x500 [i915] 
[    8.627047]  intel_display_driver_probe_noirq+0x87/0x300 [i915] 
[    8.633188]  i915_driver_probe+0x207/0x5d0 [i915] 
[    8.637977]  ? drm_privacy_screen_get+0x198/0x1c0 
[    8.642832]  local_pci_probe+0x41/0x90 
[    8.646646]  pci_call_probe+0x58/0x160 
[    8.650458]  ? pci_assign_irq+0x2f/0x160 
[    8.654447]  ? pci_match_device+0xf8/0x120 
[    8.658522]  pci_device_probe+0x95/0x140 
[    8.662582]  call_driver_probe+0x27/0x110 
[    8.666570]  really_probe+0xcc/0x2c0 
[    8.670190]  __driver_probe_device+0x78/0x120 
[    8.674692]  driver_probe_device+0x1f/0xa0 
[    8.678857]  __driver_attach+0xfa/0x230 
[    8.682757]  ? __pfx___driver_attach+0x10/0x10 
[    8.687185]  bus_for_each_dev+0x8e/0xe0 
[    8.691159]  bus_add_driver+0x11f/0x200 
[    8.694970]  driver_register+0x72/0xd0 
[    8.698853]  i915_init+0x26/0x90 [i915] 
[    8.702837]  ? __pfx_i915_init+0x10/0x10 [i915] 
[    8.707433]  do_one_initcall+0x5c/0x320 
[    8.711409]  do_init_module+0x60/0x240 
[    8.715132]  init_module_from_file+0xd6/0x130 
[    8.719634]  idempotent_init_module+0x114/0x310 
[    8.724241]  __x64_sys_finit_module+0x71/0xe0 
[    8.728671]  do_syscall_64+0x11b/0x6d0 
[    8.732483]  ? ksys_read+0x6b/0xe0 
[    8.735854]  ? arch_exit_to_user_mode_prepare.isra.0+0xa2/0xd0 
[    8.741768]  ? do_syscall_64+0x153/0x6d0 
[    8.745828]  ? do_syscall_64+0x153/0x6d0 
[    8.749814]  ? do_syscall_64+0x153/0x6d0 
[    8.753800]  ? clear_bhb_loop+0x30/0x80 
[    8.757700]  entry_SYSCALL_64_after_hwframe+0x76/0x7e 

---
 drivers/gpu/drm/i915/display/intel_dmc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dmc.c b/drivers/gpu/drm/i915/display/intel_dmc.c
index 1006b060c3f3..fd2756badc0c 100644
--- a/drivers/gpu/drm/i915/display/intel_dmc.c
+++ b/drivers/gpu/drm/i915/display/intel_dmc.c
@@ -1578,7 +1578,7 @@ void intel_dmc_update_dc6_allowed_count(struct intel_display *display,
 	struct intel_dmc *dmc = display_to_dmc(display);
 	u32 dc5_cur_count;
 
-	if (DISPLAY_VER(dmc->display) < 14)
+	if (!dmc || DISPLAY_VER(dmc->display) < 14)
 		return;
 
 	dc5_cur_count = intel_de_read(dmc->display, DG1_DMC_DEBUG_DC5_COUNT);
-- 
2.47.0