Re: [PATCH 2/5] blktrace: fix debugfs use after free

[Eric Sandeen]

On 4/15/20 7:56 PM, Luis Chamberlain wrote:
> On Wed, Apr 15, 2020 at 12:38:26PM -0500, Eric Sandeen wrote:
>> On 4/13/20 11:18 PM, Luis Chamberlain wrote:
>>> On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
>>> merged on v4.12 Omar fixed the original blktrace code for request-based
>>> drivers (multiqueue). This however left in place a possible crash, if you
>>> happen to abuse blktrace in a way it was not intended.
>>>
>>> Namely, if you loop adding a device, setup the blktrace with BLKTRACESETUP,
>>> forget to BLKTRACETEARDOWN, and then just remove the device you end up
>>> with a panic:
>>
>> I think this patch makes this all cleaner anyway, but - without the apparent
>> loop bug mentioned by Bart which allows removal of the loop device while blktrace
>> is active (if I read that right), can this still happen?
> 
> I have not tested that, but some modifications of the break-blktrace
> program could enable us to test that

FWIW, I modified it to modprobe & rmmod scsi_debug instead of the loop ioctls,
and the module can't be unloaded after the blktrace is started since it's busy.

Not sure that's equivalent tho.

> however I don't think the race
> would be possible after patch 3/5 "blktrace: refcount the request_queue
> during ioctl" is merged, as removal then a pending blktrace would
> refcount the request_queue and the removal would have to wait until
> the refcount is decremeneted, until after the blktrace ioctl.

I'm out of my depth in the block layer, not sure who's supposed to take
refs on what. ;)

-Eric