Re: [Intel-gfx] [PATCH v1] drm/i915: fix null pointer dereference

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 23 Aug 2022, Łukasz Bartosik <lb@semihalf.com> wrote:
>>
>> Hi all,
>>
>> Apologies in advance if you see this twice. I did not see the original
>> make it to either lore.kernel.org or the freedesktop.org archives so I
>> figured it might have been sent into the void.
>>
>> On Tue, Feb 01, 2022 at 04:33:54PM +0100, Lukasz Bartosik wrote:
>> > From: Łukasz Bartosik <lb@semihalf.com>
>> >
>> > Asus chromebook CX550 crashes during boot on v5.17-rc1 kernel.
>> > The root cause is null pointer defeference of bi_next
>> > in tgl_get_bw_info() in drivers/gpu/drm/i915/display/intel_bw.c.
>> >
>> > BUG: kernel NULL pointer dereference, address: 000000000000002e
>> > PGD 0 P4D 0
>> > Oops: 0002 [#1] PREEMPT SMP NOPTI
>> > CPU: 0 PID: 1 Comm: swapper/0 Tainted: G     U            5.17.0-rc1
>> > Hardware name: Google Delbin/Delbin, BIOS Google_Delbin.13672.156.3 05/14/2021
>> > RIP: 0010:tgl_get_bw_info+0x2de/0x510
>> > ...
>> > [    2.554467] Call Trace:
>> > [    2.554467]  <TASK>
>> > [    2.554467]  intel_bw_init_hw+0x14a/0x434
>> > [    2.554467]  ? _printk+0x59/0x73
>> > [    2.554467]  ? _dev_err+0x77/0x91
>> > [    2.554467]  i915_driver_hw_probe+0x329/0x33e
>> > [    2.554467]  i915_driver_probe+0x4c8/0x638
>> > [    2.554467]  i915_pci_probe+0xf8/0x14e
>> > [    2.554467]  ? _raw_spin_unlock_irqrestore+0x12/0x2c
>> > [    2.554467]  pci_device_probe+0xaa/0x142
>> > [    2.554467]  really_probe+0x13f/0x2f4
>> > [    2.554467]  __driver_probe_device+0x9e/0xd3
>> > [    2.554467]  driver_probe_device+0x24/0x7c
>> > [    2.554467]  __driver_attach+0xba/0xcf
>> > [    2.554467]  ? driver_attach+0x1f/0x1f
>> > [    2.554467]  bus_for_each_dev+0x8c/0xc0
>> > [    2.554467]  bus_add_driver+0x11b/0x1f7
>> > [    2.554467]  driver_register+0x60/0xea
>> > [    2.554467]  ? mipi_dsi_bus_init+0x16/0x16
>> > [    2.554467]  i915_init+0x2c/0xb9
>> > [    2.554467]  ? mipi_dsi_bus_init+0x16/0x16
>> > [    2.554467]  do_one_initcall+0x12e/0x2b3
>> > [    2.554467]  do_initcall_level+0xd6/0xf3
>> > [    2.554467]  do_initcalls+0x4e/0x79
>> > [    2.554467]  kernel_init_freeable+0xed/0x14d
>> > [    2.554467]  ? rest_init+0xc1/0xc1
>> > [    2.554467]  kernel_init+0x1a/0x120
>> > [    2.554467]  ret_from_fork+0x1f/0x30
>> > [    2.554467]  </TASK>
>> > ...
>> > Kernel panic - not syncing: Fatal exception
>> >
>> > Fixes: c64a9a7c05be ("drm/i915: Update memory bandwidth formulae")
>> > Signed-off-by: Łukasz Bartosik <lb@semihalf.com>
>> > ---
>> >  drivers/gpu/drm/i915/display/intel_bw.c | 16 +++++++++-------
>> >  1 file changed, 9 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/display/intel_bw.c b/drivers/gpu/drm/i915/display/intel_bw.c
>> > index 2da4aacc956b..bd0ed68b7faa 100644
>> > --- a/drivers/gpu/drm/i915/display/intel_bw.c
>> > +++ b/drivers/gpu/drm/i915/display/intel_bw.c
>> > @@ -404,15 +404,17 @@ static int tgl_get_bw_info(struct drm_i915_private *dev_priv, const struct intel
>> >               int clpchgroup;
>> >               int j;
>> >
>> > -             if (i < num_groups - 1)
>> > -                     bi_next = &dev_priv->max_bw[i + 1];
>> > -
>> >               clpchgroup = (sa->deburst * qi.deinterleave / num_channels) << i;
>> >
>> > -             if (i < num_groups - 1 && clpchgroup < clperchgroup)
>> > -                     bi_next->num_planes = (ipqdepth - clpchgroup) / clpchgroup + 1;
>> > -             else
>> > -                     bi_next->num_planes = 0;
>> > +             if (i < num_groups - 1) {
>> > +                     bi_next = &dev_priv->max_bw[i + 1];
>> > +
>> > +                     if (clpchgroup < clperchgroup)
>> > +                             bi_next->num_planes = (ipqdepth - clpchgroup) /
>> > +                                                    clpchgroup + 1;
>> > +                     else
>> > +                             bi_next->num_planes = 0;
>> > +             }
>> >
>> >               bi->num_qgv_points = qi.num_points;
>> >               bi->num_psf_gv_points = qi.num_psf_points;
>> > --
>> > 2.35.0.rc2.247.g8bbb082509-goog
>> >
>> >
>>
>> Was this patch ever applied or was the issue fixed in a different way?
>> If CONFIG_INIT_STACK_ALL_ZERO is enabled (it is on by default when the
>> compiler supports it), bi_next will be deterministically initialized to
>> NULL, which means 'bi_next->num_planes = 0' will crash when the first if
>> statement is not taken (i.e. 'i > num_groups - 1'). This was reported to
>> us at [1] so it impacts real users (and I have been applying this change
>> locally for six months). I see some discussion in this thread, was it
>> ever resolved?
>>
>> [1]: https://github.com/ClangBuiltLinux/linux/issues/1626
>>
>> Cheers,
>> Nathan
>
> The patch was not accepted by upstream. I gave up after sending two reminders
> that the issue is still present which resulted in no upstream reaction.
> I have been also applying that patch locally for a few months.
> Thanks for bringing it up to upstream attention again.

Apologies for us dropping the ball here. There were objections to the
code from Ville [1] but nobody stepped up to clean it up. I think this
was really more about the commit being fixed c64a9a7c05be ("drm/i915:
Update memory bandwidth formulae") than about the patch at hand.

In any case, I've gone ahead and pushed this patch to drm-intel-next
now. With the Fixes tag it should eventually find its way to stable
v5.17+. Thank you for the patch, review - and nagging. ;)

What still remains is cleaning up the code. But that should never have
stalled the fix for months. Sorry again.


BR,
Jani.


[1] https://lore.kernel.org/r/YgOYBfQJF7hIzEPE@intel.com

-- 
Jani Nikula, Intel Open Source Graphics Center