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

[Kees Cook]

On Mon, May 23, 2016 at 6:45 AM, Nicolai Stange <nicstange@xxxxxxxxx> 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@xxxxxxxxx>
> ---
>  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@xxxxxxxxxx>:
>  http://lkml.kernel.org/g/573F4200.3080208@xxxxxxxxxx
>
>  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