Commit f57e515a authored Jul 16, 2019 by Joel Fernandes (Google) Committed by Linus Torvalds Jul 16, 2019

kernel/pid.c: convert struct pid count to refcount_t

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.

Link: http://lkml.kernel.org/r/20190701183826.191936-1-joel@joelfernandes.org


Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Jann Horn <jannh@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: KJ Tsanaktsidis <ktsanaktsidis@zendesk.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 156e0b1a

include/linux/pid.h

+3 −2

Original line number	Diff line number	Diff line
		@@ -4,6 +4,7 @@

		#include <linux/rculist.h>
		#include <linux/wait.h>
		#include <linux/refcount.h>

		enum pid_type
		{
		@@ -57,7 +58,7 @@ 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];
		@@ -74,7 +75,7 @@ extern const struct file_operations pidfd_fops;
		static inline struct pid get_pid(struct pid pid)
		{
		if (pid)
		atomic_inc(&pid->count);
		refcount_inc(&pid->count);
		return pid;
		}

kernel/pid.c

+4 −5

Original line number	Diff line number	Diff line
		@@ -37,14 +37,14 @@
		#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/anon_inodes.h>
		#include <linux/sched/signal.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 },
		@@ -108,8 +108,7 @@ 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);
		}
		@@ -212,7 +211,7 @@ 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]);