drm/imagination: take paired job reference

For paired jobs, have the fragment job take a reference on the
geometry job, so that the geometry job cannot be freed until
the fragment job has finished with it.

The geometry job structure is accessed when the fragment job is being
prepared by the GPU scheduler. Taking the reference prevents the
geometry job being freed until the fragment job no longer requires it.

Fixes a use after free bug detected by KASAN:

[  124.256386] BUG: KASAN: slab-use-after-free in pvr_queue_prepare_job+0x108/0x868 [powervr]
[  124.264893] Read of size 1 at addr ffff0000084cb960 by task kworker/u16:4/63

Cc: stable@vger.kernel.org
Fixes: eaf01ee ("drm/imagination: Implement job submission and scheduling")
Signed-off-by: Brendan King <brendan.king@imgtec.com>
Reviewed-by: Matt Coster <matt.coster@imgtec.com>
Link: https://lore.kernel.org/r/20250318-ddkopsrc-1337-use-after-free-in-pvr_queue_prepare_job-v1-1-80fb30d044a6@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>

drivers/gpu/drm/imagination/pvr_job.c

@@ -677,6 +677,13 @@ pvr_jobs_link_geom_frag(struct pvr_job_data *job_data, u32 *job_count)
 		geom_job->paired_job = frag_job;
 		frag_job->paired_job = geom_job;
 
+		/* The geometry job pvr_job structure is used when the fragment
+		 * job is being prepared by the GPU scheduler. Have the fragment
+		 * job hold a reference on the geometry job to prevent it being
+		 * freed until the fragment job has finished with it.
+		 */
+		pvr_job_get(geom_job);
+
 		/* Skip the fragment job we just paired to the geometry job. */
 		i++;
 	}

drivers/gpu/drm/imagination/pvr_queue.c

@@ -866,6 +866,10 @@ static void pvr_queue_free_job(struct drm_sched_job *sched_job)
 	struct pvr_job *job = container_of(sched_job, struct pvr_job, base);
 
 	drm_sched_job_cleanup(sched_job);
+
+	if (job->type == DRM_PVR_JOB_TYPE_FRAGMENT && job->paired_job)
+		pvr_job_put(job->paired_job);
+
 	job->paired_job = NULL;
 	pvr_job_put(job);
 }