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

Anant Thazhemadam anant.thazhemadam at gmail.com
Sat Oct 3 22:07:54 UTC 2020

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 at syzkaller.appspotmail.com
>> Tested-by: syzbot+6ce141c55b2f7aafd1c4 at syzkaller.appspotmail.com
>> Signed-off-by: Anant Thazhemadam <anant.thazhemadam at gmail.com>
> 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:
> 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.
> 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;
>         }
> 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

I did some more investigating and testing for this bug, and turns out I was very
wrong. I'm truly sorry for that.

The memory leak that is caused is caused when !hu->serdev itself, since we free
h5, but not h5->rx_skb. This is what causes the memory leak.

I'll send in a v3 that corrects this issue soon enough, by freeing h5->rx_skb first
followed by h5 when !hu->serdev; and otherwise (when hu->serdev exists)
calling h5_reset_rx().

Sorry for the trouble.


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