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* [PATCH] kernel/kcov: unproxify debugfs file's fops
@ 2016-05-23 13:45 Nicolai Stange
  2016-05-23 18:00 ` Kees Cook
  0 siblings, 1 reply; 3+ messages in thread
From: Nicolai Stange @ 2016-05-23 13:45 UTC (permalink / raw)
  To: Greg Kroah-Hartman
  Cc: Sasha Levin, Andrew Morton, Dmitry Vyukov, Kees Cook,
	Andrey Ryabinin, Nicolai Stange, James Morse, linux-kernel

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@gmail.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@oracle.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)) {
 		pr_err("failed to create kcov in debugfs\n");
 		return -ENOMEM;
 	}
-- 
2.8.2

^ permalink raw reply related	[flat|nested] 3+ messages in thread
* Re: [PATCH] kernel/kcov: unproxify debugfs file's fops
  2016-05-23 13:45 [PATCH] kernel/kcov: unproxify debugfs file's fops Nicolai Stange
@ 2016-05-23 18:00 ` Kees Cook
  2016-05-24 12:07   ` Nicolai Stange
  0 siblings, 1 reply; 3+ messages in thread
From: Kees Cook @ 2016-05-23 18:00 UTC (permalink / raw)
  To: Nicolai Stange
  Cc: Greg Kroah-Hartman, Sasha Levin, Andrew Morton, Dmitry Vyukov,
	Andrey Ryabinin, James Morse, LKML

On Mon, May 23, 2016 at 6:45 AM, Nicolai Stange <nicstange@gmail.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@gmail.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@oracle.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.

-Kees

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



-- 
Kees Cook
Chrome OS & Brillo Security

^ permalink raw reply	[flat|nested] 3+ messages in thread
* Re: [PATCH] kernel/kcov: unproxify debugfs file's fops
  2016-05-23 18:00 ` Kees Cook
@ 2016-05-24 12:07   ` Nicolai Stange
  0 siblings, 0 replies; 3+ messages in thread
From: Nicolai Stange @ 2016-05-24 12:07 UTC (permalink / raw)
  To: Kees Cook
  Cc: Nicolai Stange, Greg Kroah-Hartman, Sasha Levin, Andrew Morton,
	Dmitry Vyukov, Andrey Ryabinin, James Morse, LKML

Kees Cook <keescook@chromium.org> writes:

> On Mon, May 23, 2016 at 6:45 AM, Nicolai Stange <nicstange@gmail.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@gmail.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@oracle.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
>>

^ permalink raw reply	[flat|nested] 3+ messages in thread
end of thread, other threads:[~2016-05-24 12:07 UTC | newest]

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2016-05-23 13:45 [PATCH] kernel/kcov: unproxify debugfs file's fops Nicolai Stange
2016-05-23 18:00 ` Kees Cook
2016-05-24 12:07   ` Nicolai Stange

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