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

[Tao Liu <ltao@redhat.com>]

On Mon, Mar 2, 2026 at 10:14 PM Jani Nikula <jani.nikula@linux.intel.com> wrote:
>
> On Sun, 01 Mar 2026, Tao Liu <ltao@redhat.com> wrote:
> > 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):
>
> The previously posted fix is [1].

Thanks for the link, I didn't notice it. For kdump case, as far as I
have tested, only adding (!dmc) check for
intel_dmc_update_dc6_allowed_count() is sufficient to allow kdump to
work.

Thanks,
Tao Liu

>
> Imre, please R-b that. It's a NULL pointer dereference in the wild, in
> stable kernels. We need to get it fixed instead of bikeshedding on
> potential incorrect debugfs results.
>
> BR,
> Jani.
>
>
> [1] https://lore.kernel.org/r/20251202183950.2450315-1-jani.nikula@intel.com
>
>
> >
> >     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);
>
> --
> Jani Nikula, Intel
>