linux-kernel - Re: [PATCH v2] kernel/kcov: unproxify debugfs file's fops

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Date:	Tue, 24 May 2016 07:39:55 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Nicolai Stange <nicstange@...il.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kernel/kcov: unproxify debugfs file's fops

On Tue, May 24, 2016 at 02:05:05PM +0200, Nicolai Stange wrote:
> Since commit 49d200deaa68 ("debugfs: prevent access to removed files'
> private data"), a debugfs file's file_operations methods get proxied
> through lifetime aware wrappers.
> 
> However, only a certain subset of the file_operations members is supported
> by debugfs and ->mmap isn't among them -- it appears to be NULL from the
> VFS layer's perspective.
> 
> This behaviour breaks the /sys/kernel/debug/kcov file introduced
> concurrently with commit 5c9a8750a640 ("kernel: add kcov code coverage").
> 
> Since that file never gets removed, there is no file removal race and thus,
> a lifetime checking proxy isn't needed.
> 
> Avoid the proxying for /sys/kernel/debug/kcov by creating it via
> debugfs_create_file_unsafe() rather than debugfs_create_file().
> 
> Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private
>                       data")
> Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
> Signed-off-by: Nicolai Stange <nicstange@...il.com>
> ---
>  The v1 thread can be found at
>  http://lkml.kernel.org/g/1464011147-31836-1-git-send-email-nicstange@gmail.com
> 
>  Changes to v1:
>  - Following the suggestion of Kees Cook, a comment explaining why the use 
>    of debugfs_create_file_unsafe() is actually safe there has been added.
> 
>  This issue has been debugged and reported by
>  Sasha Levin <sasha.levin@...cle.com>:
>  http://lkml.kernel.org/g/573F4200.3080208@oracle.com
> 
>  Applicable to linux-next 20160524.
>  In particular, it depends on
>  - c64688081490 ("debugfs: add support for self-protecting attribute file
>                   fops")
>  - 5c9a8750a640 ("kernel: add kcov code coverage")
> 
>  kernel/kcov.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index a02f2dd..8d44b3f 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -264,7 +264,12 @@ static const struct file_operations kcov_fops = {
>  
>  static int __init kcov_init(void)
>  {
> -	if (!debugfs_create_file("kcov", 0600, NULL, NULL, &kcov_fops)) {
> +	/*
> +	 * The kcov debugfs file won't ever get removed and thus,
> +	 * there is no need to protect it against removal races. The
> +	 * use of debugfs_create_file_unsafe() is actually safe here.
> +	 */
> +	if (!debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops)) {
>  		pr_err("failed to create kcov in debugfs\n");
>  		return -ENOMEM;
>  	}

Thanks, I'll queue this up after 4.7-rc1 is out.

greg k-h