gpu: nvgpu: hold ch ref when getting ch from fd

Fix a race condition in gk20a_get_channel_from_file() that returns a
channel pointer from an fd: take a reference to the channel before
putting the file ref back. Now the caller is responsible of releasing
the channel reference eventually.

Also document why dbg_session_channel_data has to hold a ref to the
channel file instead of just the channel: that might deadlock if the fds
were closed in "wrong" order.

Change-Id: I8e91b809f5f7b1cb0c1487bd955ad6d643727a53
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1549290
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/common/linux/ioctl_as.c

@@ -42,14 +42,19 @@ static int gk20a_as_ioctl_bind_channel(
 	gk20a_dbg_fn("");
 
 	ch = gk20a_get_channel_from_file(args->channel_fd);
-	if (!ch || gk20a_channel_as_bound(ch))
+	if (!ch)
 		return -EINVAL;
 
+	if (gk20a_channel_as_bound(ch)) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	/* this will set channel_gk20a->vm */
 	err = ch->g->ops.mm.vm_bind_channel(as_share, ch);
-	if (err)
-		return err;
 
+out:
+	gk20a_channel_put(ch);
 	return err;
 }
 

drivers/gpu/nvgpu/common/linux/ioctl_channel.c

@@ -260,8 +260,15 @@ static int gk20a_init_error_notifier(struct channel_gk20a *ch,
 	return 0;
 }
 
+/*
+ * This returns the channel with a reference. The caller must
+ * gk20a_channel_put() the ref back after use.
+ *
+ * NULL is returned if the channel was not found.
+ */
 struct channel_gk20a *gk20a_get_channel_from_file(int fd)
 {
+	struct channel_gk20a *ch;
 	struct channel_priv *priv;
 	struct file *f = fget(fd);
 
@@ -274,8 +281,9 @@ struct channel_gk20a *gk20a_get_channel_from_file(int fd)
 	}
 
 	priv = (struct channel_priv *)f->private_data;
+	ch = gk20a_channel_get(priv->c);
 	fput(f);
-	return priv->c;
+	return ch;
 }
 
 int gk20a_channel_release(struct inode *inode, struct file *filp)

drivers/gpu/nvgpu/common/linux/ioctl_ctrl.c

@@ -354,6 +354,8 @@ static int nvgpu_gpu_ioctl_inval_icache(
 	nvgpu_mutex_acquire(&g->dbg_sessions_lock);
 	err = g->ops.gr.inval_icache(g, ch);
 	nvgpu_mutex_release(&g->dbg_sessions_lock);
+
+	gk20a_channel_put(ch);
 	return err;
 }
 
@@ -393,6 +395,7 @@ static int nvgpu_gpu_ioctl_set_debug_mode(
 		err = -ENOSYS;
 	nvgpu_mutex_release(&g->dbg_sessions_lock);
 
+	gk20a_channel_put(ch);
 	return err;
 }
 

drivers/gpu/nvgpu/common/linux/ioctl_tsg.c

@@ -49,6 +49,8 @@ static int gk20a_tsg_bind_channel_fd(struct tsg_gk20a *tsg, int ch_fd)
 		return -EINVAL;
 
 	err = ch->g->ops.fifo.tsg_bind_channel(tsg, ch);
+
+	gk20a_channel_put(ch);
 	return err;
 }
 

drivers/gpu/nvgpu/gk20a/dbg_gpu_gk20a.c

@@ -514,7 +514,8 @@ static int dbg_unbind_channel_gk20a(struct dbg_session_gk20a *dbg_s,
 
 	if (!channel_found) {
 		gk20a_dbg_fn("channel not bounded, fd=%d\n", args->channel_fd);
-		return -EINVAL;
+		err = -EINVAL;
+		goto out;
 	}
 
 	nvgpu_mutex_acquire(&g->dbg_sessions_lock);
@@ -523,6 +524,8 @@ static int dbg_unbind_channel_gk20a(struct dbg_session_gk20a *dbg_s,
 	nvgpu_mutex_release(&dbg_s->ch_list_lock);
 	nvgpu_mutex_release(&g->dbg_sessions_lock);
 
+out:
+	gk20a_channel_put(ch);
 	return err;
 }
 
@@ -584,13 +587,16 @@ static int dbg_bind_channel_gk20a(struct dbg_session_gk20a *dbg_s,
 	struct channel_gk20a *ch;
 	struct dbg_session_channel_data *ch_data;
 	struct dbg_session_data *session_data;
+	int err = 0;
 
 	gk20a_dbg(gpu_dbg_fn|gpu_dbg_gpu_dbg, "%s fd=%d",
 		   g->name, args->channel_fd);
 
-	/* even though get_file_channel is doing this it releases it as well */
-	/* by holding it here we'll keep it from disappearing while the
-	 * debugger is in session */
+	/*
+	 * Although gk20a_get_channel_from_file gives us a channel ref, need to
+	 * hold a ref to the file during the session lifetime. See comment in
+	 * struct dbg_session_channel_data.
+	 */
 	f = fget(args->channel_fd);
 	if (!f)
 		return -ENODEV;
@@ -598,8 +604,8 @@ static int dbg_bind_channel_gk20a(struct dbg_session_gk20a *dbg_s,
 	ch = gk20a_get_channel_from_file(args->channel_fd);
 	if (!ch) {
 		gk20a_dbg_fn("no channel found for fd");
-		fput(f);
-		return -EINVAL;
+		err = -EINVAL;
+		goto out_fput;
 	}
 
 	gk20a_dbg_fn("%s hwchid=%d", g->name, ch->chid);
@@ -609,8 +615,8 @@ static int dbg_bind_channel_gk20a(struct dbg_session_gk20a *dbg_s,
 
 	ch_data = nvgpu_kzalloc(g, sizeof(*ch_data));
 	if (!ch_data) {
-		fput(f);
-		return -ENOMEM;
+		err = -ENOMEM;
+		goto out_chput;
 	}
 	ch_data->ch_f = f;
 	ch_data->channel_fd = args->channel_fd;
@@ -619,9 +625,8 @@ static int dbg_bind_channel_gk20a(struct dbg_session_gk20a *dbg_s,
 
 	session_data = nvgpu_kzalloc(g, sizeof(*session_data));
 	if (!session_data) {
-		nvgpu_kfree(g, ch_data);
-		fput(f);
-		return -ENOMEM;
+		err = -ENOMEM;
+		goto out_kfree;
 	}
 	session_data->dbg_s = dbg_s;
 	nvgpu_init_list_node(&session_data->dbg_s_entry);
@@ -636,7 +641,19 @@ static int dbg_bind_channel_gk20a(struct dbg_session_gk20a *dbg_s,
 	nvgpu_mutex_release(&ch->dbg_s_lock);
 	nvgpu_mutex_release(&g->dbg_sessions_lock);
 
+	gk20a_channel_put(ch);
+
 	return 0;
+
+out_kfree:
+	nvgpu_kfree(g, ch_data);
+out_chput:
+	gk20a_channel_put(ch);
+	nvgpu_mutex_release(&ch->dbg_s_lock);
+	nvgpu_mutex_release(&g->dbg_sessions_lock);
+out_fput:
+	fput(f);
+	return err;
 }
 
 static int nvgpu_ioctl_channel_reg_ops(struct dbg_session_gk20a *dbg_s,

drivers/gpu/nvgpu/gk20a/dbg_gpu_gk20a.h

@@ -93,6 +93,13 @@ dbg_session_data_from_dbg_s_entry(struct nvgpu_list_node *node)
 };
 
 struct dbg_session_channel_data {
+	/*
+	 * We have to keep a ref to the _file_, not the channel, because
+	 * close(channel_fd) is synchronous and would deadlock if we had an
+	 * open debug session fd holding a channel ref at that time. Holding a
+	 * ref to the file makes close(channel_fd) just drop a kernel ref to
+	 * the file; the channel will close when the last file ref is dropped.
+	 */
 	struct file *ch_f;
 	int channel_fd;
 	int chid;