drm/msm/gpu: Fix potential double-free

If userspace was calling the MSM_SET_PARAM ioctl on multiple threads to
set the COMM or CMDLINE param, it could trigger a race causing the
previous value to be kfree'd multiple times.  Fix this by serializing on
the gpu lock.
Signed-off-by: Rob Clark <robdclark@chromium.org>
Fixes: d4726d77 ("drm/msm: Add a way to override processes comm/cmdline")
Patchwork: https://patchwork.freedesktop.org/patch/517778/
Link: https://lore.kernel.org/r/20230110212903.1925878-1-robdclark@gmail.com

drivers/gpu/drm/msm/adreno/adreno_gpu.c

@@ -352,6 +352,8 @@ int adreno_set_param(struct msm_gpu *gpu, struct msm_file_private *ctx,
 		/* Ensure string is null terminated: */
 		str[len] = '\0';
 
+		mutex_lock(&gpu->lock);
+
 		if (param == MSM_PARAM_COMM) {
 			paramp = &ctx->comm;
 		} else {
@@ -361,6 +363,8 @@ int adreno_set_param(struct msm_gpu *gpu, struct msm_file_private *ctx,
 		kfree(*paramp);
 		*paramp = str;
 
+		mutex_unlock(&gpu->lock);
+
 		return 0;
 	}
 	case MSM_PARAM_SYSPROF:

drivers/gpu/drm/msm/msm_gpu.c

@@ -335,6 +335,8 @@ static void get_comm_cmdline(struct msm_gem_submit *submit, char **comm, char **
 	struct msm_file_private *ctx = submit->queue->ctx;
 	struct task_struct *task;
 
+	WARN_ON(!mutex_is_locked(&submit->gpu->lock));
+
 	/* Note that kstrdup will return NULL if argument is NULL: */
 	*comm = kstrdup(ctx->comm, GFP_KERNEL);
 	*cmd  = kstrdup(ctx->cmdline, GFP_KERNEL);

drivers/gpu/drm/msm/msm_gpu.h

@@ -366,10 +366,18 @@ struct msm_file_private {
 	 */
 	int sysprof;
 
-	/** comm: Overridden task comm, see MSM_PARAM_COMM */
+	/**
+	 * comm: Overridden task comm, see MSM_PARAM_COMM
+	 *
+	 * Accessed under msm_gpu::lock
+	 */
 	char *comm;
 
-	/** cmdline: Overridden task cmdline, see MSM_PARAM_CMDLINE */
+	/**
+	 * cmdline: Overridden task cmdline, see MSM_PARAM_CMDLINE
+	 *
+	 * Accessed under msm_gpu::lock
+	 */
 	char *cmdline;
 
 	/**