>
> Many subsystems will not use refcount_t unless there is a way to build the
> kernel so that there is no regression in speed compared to atomic_t. This
> adds CONFIG_REFCOUNT_FULL to enable the full refcount_t implementation
> which has the validation but is slightly slower. When not enabled,
> refcount_t uses the basic unchecked atomic_t routines, which results in
> no code changes compared to just using atomic_t directly.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> arch/Kconfig | 9 +++++++++
> include/linux/refcount.h | 44 ++++++++++++++++++++++++++++++++++++++++++++
> lib/refcount.c | 3 +++
> 3 files changed, 56 insertions(+)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 6c00e5b00f8b..fba3bf186728 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -867,4 +867,13 @@ config STRICT_MODULE_RWX
> config ARCH_WANT_RELAX_ORDER
> bool
>
> +config REFCOUNT_FULL
> + bool "Perform full reference count validation at the expense of
> speed"
> + help
> + Enabling this switches the refcounting infrastructure from a fast
> + unchecked atomic_t implementation to a fully state checked
> + implementation, which can be slower but provides protections
> + against various use-after-free conditions that can be used in
> + security flaw exploits.
> +
> source "kernel/gcov/Kconfig"
> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index b34aa649d204..68ecb431dbab 100644
> --- a/include/linux/refcount.h
> +++ b/include/linux/refcount.h
> @@ -41,6 +41,7 @@ static inline unsigned int refcount_read(const refcount_t *r)
> return atomic_read(&r->refs);
> }
>
> +#ifdef CONFIG_REFCOUNT_FULL
> extern __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r);
> extern void refcount_add(unsigned int i, refcount_t *r);
>
> @@ -52,6 +53,49 @@ extern void refcount_sub(unsigned int i, refcount_t *r);
>
> extern __must_check bool refcount_dec_and_test(refcount_t *r);
> extern void refcount_dec(refcount_t *r);
> +#else
> +static inline __must_check bool refcount_add_not_zero(unsigned int i,
> +
> refcount_t *r)
> +{
> + return atomic_add_return(i, &r->refs) != 0;
> +}
Maybe atomic_add_unless(&r->refs, i, 0) in order to be consistent with the below inc_not_zero implementation?
> +
> +static inline void refcount_add(unsigned int i, refcount_t *r)
> +{
> + atomic_add(i, &r->refs);
> +}
> +
> +static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
> +{
> + return atomic_add_unless(&r->refs, 1, 0);
> +}
> +
> +static inline void refcount_inc(refcount_t *r)
> +{
> + atomic_inc(&r->refs);
> +}
> +
> +static inline __must_check bool refcount_sub_and_test(unsigned int i,
> +
> refcount_t *r)
> +{
> + return atomic_sub_return(i, &r->refs) == 0;
> +}
Any reason for not using atomic_sub_and_test() here?
> +
> +static inline void refcount_sub(unsigned int i, refcount_t *r)
> +{
> + atomic_sub(i, &r->refs);
> +}
> +
> +static inline __must_check bool refcount_dec_and_test(refcount_t *r)
> +{
> + return atomic_dec_return(&r->refs) == 0;
> +}
Same here: atomic_dec_and_test()?
Best Regards,
Elena.
> +
> +static inline void refcount_dec(refcount_t *r)
> +{
> + atomic_dec(&r->refs);
> +}
> +#endif /* CONFIG_REFCOUNT_FULL */
>
> extern __must_check bool refcount_dec_if_one(refcount_t *r);
> extern __must_check bool refcount_dec_not_one(refcount_t *r);
> diff --git a/lib/refcount.c b/lib/refcount.c
> index 9f906783987e..5d0582a9480c 100644
> --- a/lib/refcount.c
> +++ b/lib/refcount.c
> @@ -37,6 +37,8 @@
> #include <linux/refcount.h>
> #include <linux/bug.h>
>
> +#ifdef CONFIG_REFCOUNT_FULL
> +
> /**
> * refcount_add_not_zero - add a value to a refcount unless it is 0
> * @i: the value to add to the refcount
> @@ -225,6 +227,7 @@ void refcount_dec(refcount_t *r)
> WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit
> 0; leaking memory.\n");
> }
> EXPORT_SYMBOL(refcount_dec);
> +#endif /* CONFIG_REFCOUNT_FULL */
>
> /**
> * refcount_dec_if_one - decrement a refcount if it is 1
> --
> 2.7.4
