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

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Date:   Tue, 17 Dec 2019 14:02:40 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Jia-Ju Bai <baijiaju1990@...il.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 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.