[PATCH RFC v2] Convert struct pid count to refcount_t

[Joel Fernandes (Google)]

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@xxxxxxxxxx
Cc: oleg@xxxxxxxxxx
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: kernel-team@xxxxxxxxxxx
Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

---
Changed to RFC to get any feedback on the memory ordering.


 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