linux-kernel - Re: [PATCH] relay: replay_file_open(): NULL ptr deref in CPU hotplugs

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Message-ID: <995450c5-a4f2-d2d1-8a44-569726eb7231@kernel.dk>
Date:   Thu, 8 Sep 2022 15:19:55 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Yichun Zhang (agentzh)" <yichun@...nresty.com>,
        linux-fsdevel@...r.kernel.org
Cc:     Colin Ian King <colin.i.king@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] relay: replay_file_open(): NULL ptr deref in CPU hotplugs

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@...nresty.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