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Subject: Re: [PATCH v2] 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: <20200304105818.11781-1-cengiz@kernel.wtf>
From:   Bob Liu <bob.liu@oracle.com>
Message-ID: <dd82137a-5152-7c93-a632-ac1766286be5@oracle.com>
Date:   Thu, 5 Mar 2020 18:13:52 +0800
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On 3/4/20 6:58 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:
> 
> Coverity static analyzer marked this as a FORWARD_NULL issue with CID
> 1460458.
> 
> ```
> /kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
> 1898            ret = 0;
> 1899            if (bt == NULL)
> 1900                    ret = blk_trace_setup_queue(q, bdev);
> 1901
> 1902            if (ret == 0) {
> 1903                    if (attr == &dev_attr_act_mask)
>>>>     CID 1460458:  Null pointer dereferences  (FORWARD_NULL)
>>>>     Dereferencing null pointer "bt".
> 1904                            bt->act_mask = value;
> 1905                    else if (attr == &dev_attr_pid)
> 1906                            bt->pid = value;
> 1907                    else if (attr == &dev_attr_start_lba)
> 1908                            bt->start_lba = value;
> 1909                    else if (attr == &dev_attr_end_lba)
> ```
> 
> Added a reassignment with RCU annotation to fix the issue.
> 
> Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")
> 
> Signed-off-by: Cengiz Can <cengiz@kernel.wtf>

This version looks good to me, thanks.
Reviewed-by: Bob Liu <bob.liu@oracle.com>

> ---
> 
>     Patch Changelog
>     * v2: Added RCU annotation to assignment
> 
>  kernel/trace/blktrace.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 4560878f0bac..ca39dc3230cb 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -1896,8 +1896,11 @@ 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 = rcu_dereference_protected(q->blk_trace,
> +				lockdep_is_held(&q->blk_trace_mutex));
> +	}
> 
>  	if (ret == 0) {
>  		if (attr == &dev_attr_act_mask)
> --
> 2.25.1
>