Re: [PATCH] blktrace: fix dereference after null check

[Bob Liu <bob.liu@oracle.com>]

On 3/3/20 9:35 PM, Steven Rostedt wrote:
> On Tue, 3 Mar 2020 21:29:08 +0800
> Bob Liu <bob.liu@oracle.com> wrote:
> 
>> 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.
> 
> Why? If ret is an error, q is still valid, and bt would just be garbage. bt
> is ignored below if ret is anything but zero. Why add an unnecessary if
> condition here?
>

Oh..yes. You are right, sorry for missed the if (ret == 0) below.

Reviewed-by: Bob Liu <bob.liu@oracle.com>

> That said, the bt assignment still needs rcu annotation:
> > 		bt = rcu_dereference_protected(q->blk_trace,
> 				lockdep_is_held(&q->blk_trace_mutex));
> 
> -- Steve
> 
> 
>>
>>> +	}
>>>
>>>  	if (ret == 0) {
>>>  		if (attr == &dev_attr_act_mask)
>>> --
>>> 2.25.1
>>>   
>