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Subject: Re: [PATCH] blktrace: fix dereference after null check
To:     Cengiz Can <cengiz@kernel.wtf>, Jens Axboe <axboe@kernel.dk>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200303073358.57799-1-cengiz@kernel.wtf>
From:   Bob Liu <bob.liu@oracle.com>
Message-ID: <ec24c6d8-617f-1460-0420-bc2ac3f346c6@oracle.com>
Date:   Tue, 3 Mar 2020 21:29:08 +0800
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On 3/3/20 3:33 PM, Cengiz Can wrote:
> There was a recent change in blktrace.c that added a RCU protection to
> `q->blk_trace` in order to fix a use-after-free issue during access.
> 
> However the change missed an edge case that can lead to dereferencing of
> `bt` pointer even when it's NULL:
> 
> ```
>         bt->act_mask = value; // bt can still be NULL here
> ```
> 
> Added a reassignment into the NULL check block to fix the issue.
> 
> Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")
> 
> Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
> ---
>  Huge thanks goes to Steven Rostedt for his assistance.
> 
>  kernel/trace/blktrace.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 4560878f0bac..29ea88f10b87 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -1896,8 +1896,10 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
>  	}
> 
>  	ret = 0;
> -	if (bt == NULL)
> +	if (bt == NULL) {
>  		ret = blk_trace_setup_queue(q, bdev);
> +		bt = q->blk_trace;

The return value 'ret' should be judged, it's wrong to set 'bt' if blk_trace_setup_queue()
return failure.

> +	}
> 
>  	if (ret == 0) {
>  		if (attr == &dev_attr_act_mask)
> --
> 2.25.1
>