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

[Kees Cook <keescook@chromium.org>]

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;

(Some email path may be clogged; Nathan Chancellor has also sent email,
but I wanted to comment too.)

This patch solves https://github.com/ClangBuiltLinux/linux/issues/1626

And it's actually fixing a wild write as well: for folks where
"i < num_groups - 1" is false and they don't get "lucky" with a NULL
deref, this is writing to some address left over on the stack. :|

-Kees

-- 
Kees Cook