Re: [PATCH v2] kref: Avoid null pointer dereference after WARN

[Jason A. Donenfeld]

On Tue, Jun 27, 2017 at 9:00 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> From: Daniel Micay <danielmicay@xxxxxxxxx>
>
> The WARN_ON() checking for a NULL release pointer was (sensibly)
> removed in commit ec48c940da6c ("kref: remove WARN_ON for NULL release
> functions") since it offered no protection at all about calling a NULL
> release pointer. However, it should instead be a BUG() since continuing
> with a NULL release pointer will lead to a NULL pointer execution
> anyway. Systems with an incorrectly set mmap_min_addr and no PXN/SMEP
> protection would be left open to executing userspace memory.
>
> The kref_put() case is extracted from PaX, and Kees Cook noted it should
> be extended to the other two cases.
>
> Comparison of my build with WARN, with nothing, and with BUG:
>
>    text            data    bss         dec       hex     filename
>   11300251        5586597 13955072    30841920  1d69c40 vmlinux.warn
>   11298136        5586597 13955072    30839805  1d693fd vmlinux.none
>   11300062        5586629 13955072    30841763  1d69ba3 vmlinux.bug
>
> Signed-off-by: Daniel Micay <danielmicay@xxxxxxxxx>
> [kees: clarify commit log, refreshed diff, moved into if statement]
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/linux/kref.h | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/include/linux/kref.h b/include/linux/kref.h
> index 29220724bf1c..651a12d2425f 100644
> --- a/include/linux/kref.h
> +++ b/include/linux/kref.h
> @@ -67,6 +67,7 @@ static inline void kref_get(struct kref *kref)
>  static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
>  {
>         if (refcount_dec_and_test(&kref->refcount)) {
> +               BUG_ON(release == NULL);
>                 release(kref);
>                 return 1;
>         }
> @@ -78,6 +79,7 @@ static inline int kref_put_mutex(struct kref *kref,
>                                  struct mutex *lock)
>  {
>         if (refcount_dec_and_mutex_lock(&kref->refcount, lock)) {
> +               BUG_ON(release == NULL);
>                 release(kref);
>                 return 1;
>         }
> @@ -89,6 +91,7 @@ static inline int kref_put_lock(struct kref *kref,
>                                 spinlock_t *lock)
>  {
>         if (refcount_dec_and_lock(&kref->refcount, lock)) {
> +               BUG_ON(release == NULL);
>                 release(kref);
>                 return 1;
>         }
> --
> 2.7.4

Looks good to me.

Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>