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SubjectRe: possible deadlock in open_rio
On Thu, 1 Aug 2019, syzbot wrote:

> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit: 7f7867ff usb-fuzzer: main usb gadget fuzzer driver
> git tree: https://github.com/google/kasan.git usb-fuzzer
> console output: https://syzkaller.appspot.com/x/log.txt?x=136b6aec600000
> kernel config: https://syzkaller.appspot.com/x/.config?x=792eb47789f57810
> dashboard link: https://syzkaller.appspot.com/bug?extid=7bbcbe9c9ff0cd49592a
> compiler: gcc (GCC) 9.0.0 20181231 (experimental)
>
> Unfortunately, I don't have any reproducer for this crash yet.
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+7bbcbe9c9ff0cd49592a@syzkaller.appspotmail.com
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.3.0-rc2+ #23 Not tainted
> ------------------------------------------------------
> syz-executor.2/20386 is trying to acquire lock:
> 00000000772249c6 (rio500_mutex){+.+.}, at: open_rio+0x16/0xc0
> drivers/usb/misc/rio500.c:64
>
> but task is already holding lock:
> 00000000d3e8f4b9 (minor_rwsem){++++}, at: usb_open+0x23/0x270
> drivers/usb/core/file.c:39
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (minor_rwsem){++++}:
> down_write+0x92/0x150 kernel/locking/rwsem.c:1500
> usb_register_dev drivers/usb/core/file.c:187 [inline]
> usb_register_dev+0x131/0x6a0 drivers/usb/core/file.c:156
> probe_rio.cold+0x53/0x21d drivers/usb/misc/rio500.c:468

This was caused by Oliver's commit 3864d33943b4 ("USB: rio500: refuse
more than one device at a time"). It added

mutex_lock(&rio500_mutex);

to probe_rio(). I guess it will be necessary to add another mutex to
fix this.

Alan Stern

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