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Message-ID: <20190624185214.GA211230@google.com>
Date: Mon, 24 Jun 2019 14:52:14 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: linux-kernel@...r.kernel.org
Cc: jannh@...gle.com, oleg@...hat.com, mathieu.desnoyers@...icios.com,
willy@...radead.org, peterz@...radead.org, will.deacon@....com,
paulmck@...ux.vnet.ibm.com, elena.reshetova@...el.com,
keescook@...omium.org, kernel-team@...roid.com,
kernel-hardening@...ts.openwall.com,
Andrew Morton <akpm@...ux-foundation.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH RFC v2] Convert struct pid count to refcount_t
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@...gle.com
> Cc: oleg@...hat.com
> Cc: mathieu.desnoyers@...icios.com
> Cc: willy@...radead.org
> Cc: peterz@...radead.org
> Cc: will.deacon@....com
> Cc: paulmck@...ux.vnet.ibm.com
> Cc: elena.reshetova@...el.com
> Cc: keescook@...omium.org
> Cc: kernel-team@...roid.com
> Cc: kernel-hardening@...ts.openwall.com
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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
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