CPU hotplug may introduce race conditions. We reproduced kernel panics
due to this NULL ptr deref error when doing frequent random CPU hotplugs
in a KVM guest.

Signed-off-by: Yichun Zhang (agentzh) <yichun@openresty.com>
---
 kernel/relay.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/relay.c b/kernel/relay.c
index 6a611e779e95..2db69fd527d1 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -816,6 +816,9 @@ EXPORT_SYMBOL_GPL(relay_flush);
 static int relay_file_open(struct inode *inode, struct file *filp)
 {
 	struct rchan_buf *buf = inode->i_private;
+	if (unlikely(buf == NULL))
+		return -ENOENT;
+
 	kref_get(&buf->kref);
 	filp->private_data = buf;
 
-- 
2.17.2

On 9/8/22 3:17 PM, Yichun Zhang (agentzh) wrote:
> CPU hotplug may introduce race conditions. We reproduced kernel panics
> due to this NULL ptr deref error when doing frequent random CPU hotplugs
> in a KVM guest.
> 
> Signed-off-by: Yichun Zhang (agentzh) <yichun@openresty.com>
> ---
>  kernel/relay.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/relay.c b/kernel/relay.c
> index 6a611e779e95..2db69fd527d1 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -816,6 +816,9 @@ EXPORT_SYMBOL_GPL(relay_flush);
>  static int relay_file_open(struct inode *inode, struct file *filp)
>  {
>  	struct rchan_buf *buf = inode->i_private;
> +	if (unlikely(buf == NULL))
> +		return -ENOENT;
> +
>  	kref_get(&buf->kref);
>  	filp->private_data = buf;

If it can go away before open is called, what prevents it from going
away after that check but before kref_get() is called?

-- 
Jens Axboe