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

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Message-ID: <87twhnek18.fsf@gmail.com>
Date:	Tue, 24 May 2016 14:07:47 +0200
From:	Nicolai Stange <nicstange@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Nicolai Stange <nicstange@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	James Morse <james.morse@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/kcov: unproxify debugfs file's fops

Kees Cook <keescook@...omium.org> writes:

> On Mon, May 23, 2016 at 6:45 AM, Nicolai Stange <nicstange@...il.com> 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>
>> ---
>>  Applicable to linux-next 20160523.
>>  In particular, it depends on
>>  - c64688081490 ("debugfs: add support for self-protecting attribute file
>>                   fops")
>>  - 5c9a8750a640 ("kernel: add kcov code coverage")
>>
>>  This issue has been debugged and reported by
>>  Sasha Levin <sasha.levin@...cle.com>:
>>  http://lkml.kernel.org/g/573F4200.3080208@oracle.com
>>
>>  kernel/kcov.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/kcov.c b/kernel/kcov.c
>> index a02f2dd..4c349dd 100644
>> --- a/kernel/kcov.c
>> +++ b/kernel/kcov.c
>> @@ -264,7 +264,7 @@ static const struct file_operations kcov_fops = {
>>
>>  static int __init kcov_init(void)
>>  {
>> -       if (!debugfs_create_file("kcov", 0600, NULL, NULL, &kcov_fops)) {
>> +       if (!debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops)) {
>
> It might make sense to add a comment above this to describe why
> "unsafe" is not unsafe in this case.

Done. v2 can be found at
http://lkml.kernel.org/g/1464091505-20943-1-git-send-email-nicstange@gmail.com

Thanks,

Nicolai

>
> -Kees
>
>>                 pr_err("failed to create kcov in debugfs\n");
>>                 return -ENOMEM;
>>         }
>> --
>> 2.8.2
>>