Re: [PATCH] kcov: convert kcov.refcount to refcount_t

[Dmitry Vyukov]

On Mon, Jan 21, 2019 at 1:38 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Wed, Jan 16, 2019 at 12:27:09PM +0200, Elena Reshetova wrote:
> > atomic_t variables are currently used to implement reference
> > counters with the following properties:
> >  - counter is initialized to 1 using atomic_set()
> >  - a resource is freed upon counter reaching zero
> >  - once counter reaches zero, its further
> >    increments aren't allowed
> >  - counter schema uses basic atomic operations
> >    (set, inc, inc_not_zero, dec_and_test, etc.)
> >
> > Such atomic variables should be converted to a newly provided
> > refcount_t type and API that prevents accidental counter overflows
> > and underflows. This is important since overflows and underflows
> > can lead to use-after-free situation and be exploitable.
> >
> > The variable kcov.refcount is used as pure reference counter.
> > Convert it to refcount_t and fix up the operations.
> >
> > **Important note for maintainers:
> >
> > Some functions from refcount_t API defined in lib/refcount.c
> > have different memory ordering guarantees than their atomic
> > counterparts.
> > The full comparison can be seen in
> > https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
> > in state to be merged to the documentation tree.
> > Normally the differences should not matter since refcount_t provides
> > enough guarantees to satisfy the refcounting use cases, but in
> > some rare cases it might matter.
> > Please double check that you don't have some undocumented
> > memory guarantees for this variable usage.
> >
> > For the kcov.refcount it might make a difference
> > in following places:
> >  - kcov_put(): decrement in refcount_dec_and_test() only
> >    provides RELEASE ordering and control dependency on success
> >    vs. fully ordered atomic counterpart
> >
> > Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
> > Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
>
> Just to check, has this been tested with CONFIG_REFCOUNT_FULL and
> something poking kcov?
>
> Given lib/refcount.c is instrumented, the refcount_*() calls will
> recurse back into the kcov code. It looks like that's fine, given these
> are only manipulated in setup/teardown paths, but it would be nice to be
> sure.

A simple program using KCOV is available here:
https://elixir.bootlin.com/linux/v5.0-rc3/source/Documentation/dev-tools/kcov.rst#L42
or here (it's like strace but collects and prints KCOV coverage):
https://github.com/google/syzkaller/blob/master/tools/kcovtrace/kcovtrace.c

> Thanks,
> Mark.
>
> > ---
> >  kernel/kcov.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/kcov.c b/kernel/kcov.c
> > index c2277db..051e86e 100644
> > --- a/kernel/kcov.c
> > +++ b/kernel/kcov.c
> > @@ -20,6 +20,7 @@
> >  #include <linux/debugfs.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/kcov.h>
> > +#include <linux/refcount.h>
> >  #include <asm/setup.h>
> >
> >  /* Number of 64-bit words written per one comparison: */
> > @@ -44,7 +45,7 @@ struct kcov {
> >        *  - opened file descriptor
> >        *  - task with enabled coverage (we can't unwire it from another task)
> >        */
> > -     atomic_t                refcount;
> > +     refcount_t              refcount;
> >       /* The lock protects mode, size, area and t. */
> >       spinlock_t              lock;
> >       enum kcov_mode          mode;
> > @@ -228,12 +229,12 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_switch);
> >
> >  static void kcov_get(struct kcov *kcov)
> >  {
> > -     atomic_inc(&kcov->refcount);
> > +     refcount_inc(&kcov->refcount);
> >  }
> >
> >  static void kcov_put(struct kcov *kcov)
> >  {
> > -     if (atomic_dec_and_test(&kcov->refcount)) {
> > +     if (refcount_dec_and_test(&kcov->refcount)) {
> >               vfree(kcov->area);
> >               kfree(kcov);
> >       }
> > @@ -312,7 +313,7 @@ static int kcov_open(struct inode *inode, struct file *filep)
> >       if (!kcov)
> >               return -ENOMEM;
> >       kcov->mode = KCOV_MODE_DISABLED;
> > -     atomic_set(&kcov->refcount, 1);
> > +     refcount_set(&kcov->refcount, 1);
> >       spin_lock_init(&kcov->lock);
> >       filep->private_data = kcov;
> >       return nonseekable_open(inode, filep);
> > --
> > 2.7.4
> >