[PATCH v3] Convert struct pid count to refcount_t

From: Joel Fernandes (Google)
Date: Mon Jul 01 2019 - 14:38:34 EST


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: mathieu.desnoyers@xxxxxxxxxxxx
Cc: willy@xxxxxxxxxxxxx
Cc: peterz@xxxxxxxxxxxxx
Cc: will.deacon@xxxxxxx
Cc: paulmck@xxxxxxxxxxxxxxxxxx
Cc: elena.reshetova@xxxxxxxxx
Cc: keescook@xxxxxxxxxxxx
Cc: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
Cc: jannh@xxxxxxxxxx
Reviewed-by: keescook@xxxxxxxxxxxx
Reviewed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

---
v1->v2 is to get rid of the atomic_read().
v2->v3 replaces ATOMIC_INIT with REFCOUNT_INIT

include/linux/pid.h | 5 +++--
kernel/pid.c | 9 ++++-----
2 files changed, 7 insertions(+), 7 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..86b526bd59e1 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -37,12 +37,12 @@
#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>

struct pid init_struct_pid = {
- .count = ATOMIC_INIT(1),
+ .count = REFCOUNT_INIT(1),
.tasks = {
{ .first = NULL },
{ .first = NULL },
@@ -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