[alsa-devel] [PATCH 1/2] ALSA: info: Fix racy addition/deletion of nodes

[Takashi Iwai]

The ALSA proc helper manages the child nodes in a linked list, but its
addition and deletion is done without any lock.  This leads to a
corruption if they are operated concurrently.  Usually this isn't a
problem because the proc entries are added sequentially in the driver
probe procedure itself.  But the card registrations are done often
asynchronously, and the crash could be actually reproduced with
syzkaller.

This patch papers over it by protecting the link addition and deletion
with the parent's mutex.  There is "access" mutex that is used for the
file access, and this can be reused for this purpose as well.

Reported-by: syzbot+48df349490c36f9f54ab at syzkaller.appspotmail.com
Cc: <stable at vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/info.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/sound/core/info.c b/sound/core/info.c
index 96a074019c33..0eb169acc850 100644
--- a/sound/core/info.c
+++ b/sound/core/info.c
@@ -713,8 +713,11 @@ snd_info_create_entry(const char *name, struct snd_info_entry *parent,
 	INIT_LIST_HEAD(&entry->list);
 	entry->parent = parent;
 	entry->module = module;
-	if (parent)
+	if (parent) {
+		mutex_lock(&parent->access);
 		list_add_tail(&entry->list, &parent->children);
+		mutex_unlock(&parent->access);
+	}
 	return entry;
 }
 
@@ -792,7 +795,12 @@ void snd_info_free_entry(struct snd_info_entry * entry)
 	list_for_each_entry_safe(p, n, &entry->children, list)
 		snd_info_free_entry(p);
 
-	list_del(&entry->list);
+	p = entry->parent;
+	if (p) {
+		mutex_lock(&p->access);
+		list_del(&entry->list);
+		mutex_unlock(&p->access);
+	}
 	kfree(entry->name);
 	if (entry->private_free)
 		entry->private_free(entry);
-- 
2.16.4