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

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Date:	Tue, 14 Jun 2016 17:19:13 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nicolai Stange <nicstange@...il.com>
Cc:	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 05/24/2016 10:39 AM, Greg Kroah-Hartman wrote:
> 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.

Hey Greg,

Just wanted to remind you about this one.


Thanks,
Sasha