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

Greg Kroah-Hartman
2016-05-24T14:39:59Z

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@gmail.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@oracle.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