[PATCH] 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. No change in behavior if
CONFIG_REFCOUNT_FULL=n.

Cc: keesc...@chromium.org
Cc: kernel-t...@android.com
Cc: kernel-harden...@lists.openwall.com
Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org>

---
 include/linux/pid.h | 5 +++--
 kernel/pid.c        | 8 ++++----
 2 files changed, 7 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..2095c7da644d 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 +106,8 @@ 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_read(&pid->count) == 1) ||
+            refcount_dec_and_test(&pid->count)) {
                kmem_cache_free(ns->pid_cachep, pid);
                put_pid_ns(ns);
        }
@@ -210,7 +210,7 @@ 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]);
 
-- 
2.21.0.392.gf8f6787159e-goog