ALSA: core: Add async signal helpers

Currently the call of kill_fasync() from an interrupt handler might
lead to potential spin deadlocks, as spotted by syzkaller.
Unfortunately, it's not so trivial to fix this lock chain as it's
involved with the tasklist_lock that is touched in allover places.

As a temporary workaround, this patch provides the way to defer the
async signal notification in a work.  The new helper functions,
snd_fasync_helper() and snd_kill_faync() are replacements for
fasync_helper() and kill_fasync(), respectively.  In addition,
snd_fasync_free() needs to be called at the destructor of the relevant
file object.

Link: https://lore.kernel.org/r/20220728125945.29533-2-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/core.h

@@ -507,4 +507,12 @@ snd_pci_quirk_lookup_id(u16 vendor, u16 device,
 }
 #endif
 
+/* async signal helpers */
+struct snd_fasync;
+
+int snd_fasync_helper(int fd, struct file *file, int on,
+		      struct snd_fasync **fasyncp);
+void snd_kill_fasync(struct snd_fasync *fasync, int signal, int poll);
+void snd_fasync_free(struct snd_fasync *fasync);
+
 #endif /* __SOUND_CORE_H */

sound/core/misc.c

@@ -10,6 +10,7 @@
 #include <linux/time.h>
 #include <linux/slab.h>
 #include <linux/ioport.h>
+#include <linux/fs.h>
 #include <sound/core.h>
 
 #ifdef CONFIG_SND_DEBUG
@@ -145,3 +146,96 @@ snd_pci_quirk_lookup(struct pci_dev *pci, const struct snd_pci_quirk *list)
 }
 EXPORT_SYMBOL(snd_pci_quirk_lookup);
 #endif
+
+/*
+ * Deferred async signal helpers
+ *
+ * Below are a few helper functions to wrap the async signal handling
+ * in the deferred work.  The main purpose is to avoid the messy deadlock
+ * around tasklist_lock and co at the kill_fasync() invocation.
+ * fasync_helper() and kill_fasync() are replaced with snd_fasync_helper()
+ * and snd_kill_fasync(), respectively.  In addition, snd_fasync_free() has
+ * to be called at releasing the relevant file object.
+ */
+struct snd_fasync {
+	struct fasync_struct *fasync;
+	int signal;
+	int poll;
+	int on;
+	struct list_head list;
+};
+
+static DEFINE_SPINLOCK(snd_fasync_lock);
+static LIST_HEAD(snd_fasync_list);
+
+static void snd_fasync_work_fn(struct work_struct *work)
+{
+	struct snd_fasync *fasync;
+
+	spin_lock_irq(&snd_fasync_lock);
+	while (!list_empty(&snd_fasync_list)) {
+		fasync = list_first_entry(&snd_fasync_list, struct snd_fasync, list);
+		list_del_init(&fasync->list);
+		spin_unlock_irq(&snd_fasync_lock);
+		if (fasync->on)
+			kill_fasync(&fasync->fasync, fasync->signal, fasync->poll);
+		spin_lock_irq(&snd_fasync_lock);
+	}
+	spin_unlock_irq(&snd_fasync_lock);
+}
+
+static DECLARE_WORK(snd_fasync_work, snd_fasync_work_fn);
+
+int snd_fasync_helper(int fd, struct file *file, int on,
+		      struct snd_fasync **fasyncp)
+{
+	struct snd_fasync *fasync = NULL;
+
+	if (on) {
+		fasync = kzalloc(sizeof(*fasync), GFP_KERNEL);
+		if (!fasync)
+			return -ENOMEM;
+		INIT_LIST_HEAD(&fasync->list);
+	}
+
+	spin_lock_irq(&snd_fasync_lock);
+	if (*fasyncp) {
+		kfree(fasync);
+		fasync = *fasyncp;
+	} else {
+		if (!fasync) {
+			spin_unlock_irq(&snd_fasync_lock);
+			return 0;
+		}
+		*fasyncp = fasync;
+	}
+	fasync->on = on;
+	spin_unlock_irq(&snd_fasync_lock);
+	return fasync_helper(fd, file, on, &fasync->fasync);
+}
+EXPORT_SYMBOL_GPL(snd_fasync_helper);
+
+void snd_kill_fasync(struct snd_fasync *fasync, int signal, int poll)
+{
+	unsigned long flags;
+
+	if (!fasync || !fasync->on)
+		return;
+	spin_lock_irqsave(&snd_fasync_lock, flags);
+	fasync->signal = signal;
+	fasync->poll = poll;
+	list_move(&fasync->list, &snd_fasync_list);
+	schedule_work(&snd_fasync_work);
+	spin_unlock_irqrestore(&snd_fasync_lock, flags);
+}
+EXPORT_SYMBOL_GPL(snd_kill_fasync);
+
+void snd_fasync_free(struct snd_fasync *fasync)
+{
+	if (!fasync)
+		return;
+	fasync->on = 0;
+	flush_work(&snd_fasync_work);
+	kfree(fasync);
+}
+EXPORT_SYMBOL_GPL(snd_fasync_free);