drm/v3d: Fix out-of-bounds read in `v3d_csd_job_run()`

When enabling UBSAN on Raspberry Pi 5, we get the following warning:

[  387.894977] UBSAN: array-index-out-of-bounds in drivers/gpu/drm/v3d/v3d_sched.c:320:3
[  387.903868] index 7 is out of range for type '__u32 [7]'
[  387.909692] CPU: 0 PID: 1207 Comm: kworker/u16:2 Tainted: G        WC         6.10.3-v8-16k-numa #151
[  387.919166] Hardware name: Raspberry Pi 5 Model B Rev 1.0 (DT)
[  387.925961] Workqueue: v3d_csd drm_sched_run_job_work [gpu_sched]
[  387.932525] Call trace:
[  387.935296]  dump_backtrace+0x170/0x1b8
[  387.939403]  show_stack+0x20/0x38
[  387.942907]  dump_stack_lvl+0x90/0xd0
[  387.946785]  dump_stack+0x18/0x28
[  387.950301]  __ubsan_handle_out_of_bounds+0x98/0xd0
[  387.955383]  v3d_csd_job_run+0x3a8/0x438 [v3d]
[  387.960707]  drm_sched_run_job_work+0x520/0x6d0 [gpu_sched]
[  387.966862]  process_one_work+0x62c/0xb48
[  387.971296]  worker_thread+0x468/0x5b0
[  387.975317]  kthread+0x1c4/0x1e0
[  387.978818]  ret_from_fork+0x10/0x20
[  387.983014] ---[ end trace ]---

This happens because the UAPI provides only seven configuration
registers and we are reading the eighth position of this u32 array.

Therefore, fix the out-of-bounds read in `v3d_csd_job_run()` by
accessing only seven positions on the '__u32 [7]' array. The eighth
register exists indeed on V3D 7.1, but it isn't currently used. That
being so, let's guarantee that it remains unused and add a note that it
could be set in a future patch.

Fixes: 0ad5bc1c ("drm/v3d: fix up register addresses for V3D 7.x")
Reported-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809152001.668314-1-mcanal@igalia.com

drivers/gpu/drm/v3d/v3d_sched.c

@@ -315,7 +315,7 @@ v3d_csd_job_run(struct drm_sched_job *sched_job)
 	struct v3d_dev *v3d = job->base.v3d;
 	struct drm_device *dev = &v3d->drm;
 	struct dma_fence *fence;
-	int i, csd_cfg0_reg, csd_cfg_reg_count;
+	int i, csd_cfg0_reg;
 
 	v3d->csd_job = job;
 
@@ -335,9 +335,17 @@ v3d_csd_job_run(struct drm_sched_job *sched_job)
 	v3d_switch_perfmon(v3d, &job->base);
 
 	csd_cfg0_reg = V3D_CSD_QUEUED_CFG0(v3d->ver);
-	csd_cfg_reg_count = v3d->ver < 71 ? 6 : 7;
-	for (i = 1; i <= csd_cfg_reg_count; i++)
+	for (i = 1; i <= 6; i++)
 		V3D_CORE_WRITE(0, csd_cfg0_reg + 4 * i, job->args.cfg[i]);
+
+	/* Although V3D 7.1 has an eighth configuration register, we are not
+	 * using it. Therefore, make sure it remains unused.
+	 *
+	 * XXX: Set the CFG7 register
+	 */
+	if (v3d->ver >= 71)
+		V3D_CORE_WRITE(0, V3D_V7_CSD_QUEUED_CFG7, 0);
+
 	/* CFG0 write kicks off the job. */
 	V3D_CORE_WRITE(0, csd_cfg0_reg, job->args.cfg[0]);