Re: [Linux-kernel-mentees][PATCH v2] bluetooth: hci_h5: close serdev device and free hu in h5_close

[Anant Thazhemadam]

On 02/10/20 3:52 pm, Hans de Goede wrote:
> Hi,
>
> On 10/1/20 9:43 PM, Anant Thazhemadam wrote:
>> When h5_close() gets called, the memory allocated for the hu gets
>> freed only if hu->serdev doesn't exist. This leads to a memory leak.
>> So when h5_close() is requested, close the serdev device instance and
>> free the memory allocated to the hu entirely instead.
>>
>> Fixes: ce945552fde4 ("Bluetooth: hci_h5: Add support for serdev enumerated devices")
>> Reported-by: syzbot+6ce141c55b2f7aafd1c4@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Tested-by: syzbot+6ce141c55b2f7aafd1c4@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@xxxxxxxxx>
>
> So 2 comments to this:
>
> 1. You claim this change fixes a memory-leak, but in the serdev case
> the memory is allocated in h5_serdev_probe() like this:
>
>        h5 = devm_kzalloc(dev, sizeof(*h5), GFP_KERNEL);
>
> So its lifetime is tied to the lifetime of the driver being bound
> to the serdev and it is automatically freed when the driver gets
> unbound. If you had looked at where the h5 struct gets allocated
> in h5_close()-s counterpart, h5_open() then you could have seen
> this there:
>
>         if (hu->serdev) {
>                 h5 = serdev_device_get_drvdata(hu->serdev);
>         } else {
>                 h5 = kzalloc(sizeof(*h5), GFP_KERNEL);
>                 if (!h5)
>                         return -ENOMEM;
>         }
>
> So it is very clear here, that the kzalloc only happens in the
> if (!hu->serdev) which clearly makes the kfree() have the same
> condition the right thing todo. In the hu->serdev the data which
> was allocated by h5_serdev_probe() is used instead and no alloc
> happens inside h5_open()
>
> In general when looking at resource teardown you should also look
> at where they are allocated in the mirror function
> and the teardown should mirror the alloc code.
>
> So the main change of your commit is wrong:
>
> NACK.
>
>
> 2. You are making multiple changes in a single commit here, I count at
> least 3. When ever you are making multiple changes like this, you should really
> split the commit up in 1 commit per change and explain in each commit
> message why that change is being made (why it is necessary). Writing
> commit messages like this also leads to you double-checking your own
> work by carefully considering why you are making the changes.
>
> So about the 3 different changes:
>
> 2a) Make the kfree(h5) call unconditionally, which as mentioned above
> is wrong.
>
Hmm, I understand, thank you.

> 2b) Introduce a call to kfree_skb(h5->rx_skb); which is not mentioned in
> the commit message at all.  This looks like it would actually be a sensible
> change, at least in the "if (!hu->serdev)" case.  When using the serdev
> interface then just freeing it will leave a dangling pointer, so it
> would be better (for both the hu->serdev and the !hu->serdev cases)
> to call h5_reset_rx() on close instead which does:
>
>         if (h5->rx_skb) {
>                 kfree_skb(h5->rx_skb);
>                 h5->rx_skb = NULL;
>         }
>
This was the first thing I did! (and I tested and it works too)
        https://syzkaller.appspot.com/text?tag=Patch&x=13acf917900000

However, I was worried that doing so might mean freeing an skb
that might be required somewhere by the serdev connection (which could
possibly lead to a null ptr dereference), so I thought closing the connection
altogether might be a better alternative.

But now, since I've been told better, I'll submit a v3, that makes the more
sensible change and doesn't close the serdev_device, with a much more
informative commit message!

I also found a similar implementation in mrvl_close() where the serdev_device
was being closed and felt it might be the right way to go. But I now know that
I shouldn't have overlooked the fact that the open functions for both differ.
I'm sorry for this.

Like you mentioned, I should've given more priority to the mirror function,
and looked to see if even they were along the same lines, and I will be sure to
keep that in mind.

> 2c) Introduce a call to serdev_device_close(), without really explaining
> why in the commit message. Again if you would have looked at the mirror
> h5_close() function then you see no serdev_device_open() there.
> Actually serdev_device_open() is not called anywhere in the hci_h5.c code.
>
> Digging a little deeper (using grep) shows that hci_uart_register_device()
> calls serdev_device_open() and hci_uart_register_device() gets called from:
> h5_serdev_probe(), likewise hci_uart_unregister_device() calls
> serdev_device_close() for us and that gets called from h5_serdev_remove(),
> so there is no need to call serdev_device_close() from h5_close() and doing
> so will actually break things, because then the serdev will be left closed
> on a subsequent h5_open() call.
>
> Regards,
>
> Hans
>
Thank you so much for this review, and for your time! :)

Thanks,
Anant