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

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Message-ID: <20180621034335.GT30522@ZenIV.linux.org.uk>
Date:   Thu, 21 Jun 2018 04:43:36 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     akpm@...ux-foundation.org, dvyukov@...gle.com,
        Greg KH <gregkh@...uxfoundation.org>, tchibo@...gle.com,
        aryabinin@...tuozzo.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] kernel: kcov: a possible sleep-in-atomic-context bug in
 kcov_ioctl()

On Thu, Jun 21, 2018 at 11:20:59AM +0800, Jia-Ju Bai wrote:
> The kernel may sleep with holding a spinlock.
> The function call path (from bottom to top) in Linux-4.16.7 is:
> 
> [FUNC] vfree --> can sleep
> kernel/kcov.c, 237: vfree in kcov_put
> kernel/kcov.c, 396: kcov_put in kcov_ioctl_locked
> kernel/kcov.c, 410: kcov_ioctl_locked in kcov_ioctl
> kernel/kcov.c, 409: spin_lock in kcov_ioctl
> 
> This bug is found by my static analysis tool (DSAC-2) and checked by my
> code review.
> 
> I do not know how to correctly fix this bug, so I just report them.

Assuming it's a bug in the first place, that is.  Note that
	* we never modify task->kcov for task != current
	* task->kcov contributes to refcount
	* opened file contributes to refcount
	* that kcov_put() of yours happens from ->ioctl() and removes
current->kcov reference; it *can't* be the last reference - the one
held by struct file used to call ->ioctl() is also there.

IOW, it's a false positive.