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

Sasha Levin
2016-06-14T21:19:33Z

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@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.

Hey Greg,

Just wanted to remind you about this one.

Thanks,
Sasha