drm/i915/sseu: fix max_subslices array-index-out-of-bounds access

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/sseu: fix max_subslices array-index-out-of-bounds access

It seems that commit bc3c5e0809ae ("drm/i915/sseu: Don't try to store EU

mask internally in UAPI format") exposed a potential out-of-bounds

access, reported by UBSAN as following on a laptop with a gen 11 i915

card:

UBSAN: array-index-out-of-bounds in drivers/gpu/drm/i915/gt/intel_sseu.c:65:27

index 6 is out of range for type 'u16 [6]'

CPU: 2 PID: 165 Comm: systemd-udevd Not tainted 6.2.0-9-generic #9-Ubuntu

Hardware name: Dell Inc. XPS 13 9300/077Y9N, BIOS 1.11.0 03/22/2022

Call Trace:

<TASK>

show_stack+0x4e/0x61

dump_stack_lvl+0x4a/0x6f

dump_stack+0x10/0x18

ubsan_epilogue+0x9/0x3a

__ubsan_handle_out_of_bounds.cold+0x42/0x47

gen11_compute_sseu_info+0x121/0x130 [i915]

intel_sseu_info_init+0x15d/0x2b0 [i915]

intel_gt_init_mmio+0x23/0x40 [i915]

i915_driver_mmio_probe+0x129/0x400 [i915]

? intel_gt_probe_all+0x91/0x2e0 [i915]

i915_driver_probe+0xe1/0x3f0 [i915]

? drm_privacy_screen_get+0x16d/0x190 [drm]

? acpi_dev_found+0x64/0x80

i915_pci_probe+0xac/0x1b0 [i915]

...

According to the definition of sseu_dev_info, eu_mask->hsw is limited to

a maximum of GEN_MAX_SS_PER_HSW_SLICE (6) sub-slices, but

gen11_sseu_info_init() can potentially set 8 sub-slices, in the

!IS_JSL_EHL(gt->i915) case.

Fix this by reserving up to 8 slots for max_subslices in the eu_mask

struct.

(cherry picked from commit 3cba09a6ac86ea1d456909626eb2685596c07822)