linux-kernel - Re: [BUG] kernel: kcov: a possible sleep-in-atomic-context bug in kcov

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Date:   Tue, 17 Dec 2019 21:17:19 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Anders Roxell <anders.roxell@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] kernel: kcov: a possible sleep-in-atomic-context bug in
 kcov_ioctl()



On 2019/12/17 21:13, Dmitry Vyukov wrote:
> On Tue, Dec 17, 2019 at 2:11 PM Jia-Ju Bai <baijiaju1990@...il.com> wrote:
>> On 2019/12/17 21:02, Dmitry Vyukov wrote:
>>> On Tue, Dec 17, 2019 at 1:56 PM Jia-Ju Bai <baijiaju1990@...il.com> wrote:
>>>> The kernel may sleep while holding a spinlock.
>>>> The function call path (from bottom to top) in Linux 4.19 is:
>>>>
>>>> kernel/kcov.c, 237:
>>>>        vfree in kcov_put
>>>> kernel/kcov.c, 413:
>>>>        kcov_put in kcov_ioctl_locked
>>>> kernel/kcov.c, 427:
>>>>        kcov_ioctl_locked in kcov_ioctl
>>>> kernel/kcov.c, 426:
>>>>        spin_lock in kcov_ioctl
>>>>
>>>> vfree() can sleep at runtime.
>>>>
>>>> I am not sure how to properly fix this possible bug, so I only report it.
>>>> A possible way is to replace vfree() with kfree(), and replace related
>>>> calls to vmalloc() with kmalloc().
>>>>
>>>> This bug is found by a static analysis tool STCheck written by myself.
>>> Hi Jia-Ju,
>>>
>>> Are you sure kcov_ioctl_locked can really release the descriptor? It
>>> happens in the context of ioctl, which means there is an open
>>> reference for the file descriptor. So ioctl should not do vfree I
>>> would assume.
>> Thanks for the reply :)
>> I am not sure, because I am not familiar with kcov.
>> But looking at the code, if the reference count of kcov is 1, vfree()
>> could be called.
> That kcov_put should never call vfree. We still hold reference
> associated with the file, which will be released in kcov_close.

Okay, thanks for the explanation :)
My static analysis tool does not know this fact, so it reports the false 
bug, sorry...


Best wishes,
Jia-Ju Bai