Re: [PATCH RFC v2] Convert struct pid count to refcount_t

Joel Fernandes
2019-06-24T18:52:19Z

On Mon, Jun 24, 2019 at 02:45:34PM -0400, Joel Fernandes (Google) wrote:
> struct pid's count is an atomic_t field used as a refcount. Use
> refcount_t for it which is basically atomic_t but does additional
> checking to prevent use-after-free bugs.
>
> For memory ordering, the only change is with the following:
> - if ((atomic_read(&pid->count) == 1) ||
> - atomic_dec_and_test(&pid->count)) {
> + if (refcount_dec_and_test(&pid->count)) {
> kmem_cache_free(ns->pid_cachep, pid);
>
> Here the change is from:
> Fully ordered --> RELEASE + ACQUIRE (as per refcount-vs-atomic.rst)
> This ACQUIRE should take care of making sure the free happens after the
> refcount_dec_and_test().
>
> The above hunk also removes atomic_read() since it is not needed for the
> code to work and it is unclear how beneficial it is. The removal lets
> refcount_dec_and_test() check for cases where get_pid() happened before
> the object was freed.
>
> Cc: jannh@google.com
> Cc: oleg@redhat.com
> Cc: mathieu.desnoyers@efficios.com
> Cc: willy@infradead.org
> Cc: peterz@infradead.org
> Cc: will.deacon@arm.com
> Cc: paulmck@linux.vnet.ibm.com
> Cc: elena.reshetova@intel.com
> Cc: keescook@chromium.org
> Cc: kernel-team@android.com
> Cc: kernel-hardening@lists.openwall.com
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> ---
> Changed to RFC to get any feedback on the memory ordering.

I had a question about refcount_inc().

As per Documentation/core-api/refcount-vs-atomic.rst , it says:

A control dependency (on success) for refcounters guarantees that
if a reference for an object was successfully obtained (reference
counter increment or addition happened, function returned true),
then further stores are ordered against this operation.

However, in refcount_inc() I don't see any memory barriers (in the case where
CONFIG_REFCOUNT_FULL=n). Is the documentation wrong?

get_pid() does a refcount_inc() but doesn't have any memory barriers either.

thanks,

- Joel

>
> include/linux/pid.h | 5 +++--
> kernel/pid.c | 7 +++----
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 14a9a39da9c7..8cb86d377ff5 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -3,6 +3,7 @@
> #define _LINUX_PID_H
>
> #include <linux/rculist.h>
> +#include <linux/refcount.h>
>
> enum pid_type
> {
> @@ -56,7 +57,7 @@ struct upid {
>
> struct pid
> {
> - atomic_t count;
> + refcount_t count;
> unsigned int level;
> /* lists of tasks that use this pid */
> struct hlist_head tasks[PIDTYPE_MAX];
> @@ -69,7 +70,7 @@ extern struct pid init_struct_pid;
> static inline struct pid *get_pid(struct pid *pid)
> {
> if (pid)
> - atomic_inc(&pid->count);
> + refcount_inc(&pid->count);
> return pid;
> }
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 20881598bdfa..89c4849fab5d 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -37,7 +37,7 @@
> #include <linux/init_task.h>
> #include <linux/syscalls.h>
> #include <linux/proc_ns.h>
> -#include <linux/proc_fs.h>
> +#include <linux/refcount.h>
> #include <linux/sched/task.h>
> #include <linux/idr.h>
>
> @@ -106,8 +106,7 @@ void put_pid(struct pid *pid)
> return;
>
> ns = pid->numbers[pid->level].ns;
> - if ((atomic_read(&pid->count) == 1) ||
> - atomic_dec_and_test(&pid->count)) {
> + if (refcount_dec_and_test(&pid->count)) {
> kmem_cache_free(ns->pid_cachep, pid);
> put_pid_ns(ns);
> }
> @@ -210,7 +209,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> }
>
> get_pid_ns(ns);
> - atomic_set(&pid->count, 1);
> + refcount_set(&pid->count, 1);
> for (type = 0; type < PIDTYPE_MAX; ++type)
> INIT_HLIST_HEAD(&pid->tasks[type]);
>
> --
> 2.22.0.410.gd8fdbe21b5-goog