sched/core: Convert signal_struct.sigcnt to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:

 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable signal_struct.sigcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

** Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.

The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.

Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.

Please double check that you don't have some undocumented
memory guarantees for this variable usage.

For the signal_struct.sigcnt it might make a difference
in following places:

 - put_signal_struct(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and control dependency on success
   vs. fully ordered atomic counterpart

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akpm@linux-foundation.org
Cc: viro@zeniv.linux.org.uk
Link: https://lkml.kernel.org/r/1547814450-18902-3-git-send-email-elena.reshetova@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 37eeb1a28eba0f7e4d445bf18688104eadf7dad2..ae56551976983dc508d3f3d63903d3a284a5f3a4 100644 (file)
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -83,7 +83,7 @@ struct multiprocess_signals {
  * the locking of signal_struct.
  */
 struct signal_struct {
-       atomic_t                sigcnt;
+       refcount_t              sigcnt;
        atomic_t                live;
        int                     nr_threads;
        struct list_head        thread_head;

diff --git a/init/init_task.c b/init/init_task.c
index 5aebe3be4d7cd65ce1bed995c76b9293fea00b2d..9aa3ebc74970a569ab3ac3582fba860ffcb53c3e 100644 (file)
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -44,7 +44,7 @@ static struct signal_struct init_signals = {
 };
 
 static struct sighand_struct init_sighand = {
-       .count          = ATOMIC_INIT(1),
+       .count          = REFCOUNT_INIT(1),
        .action         = { { { .sa_handler = SIG_DFL, } }, },
        .siglock        = __SPIN_LOCK_UNLOCKED(init_sighand.siglock),
        .signalfd_wqh   = __WAIT_QUEUE_HEAD_INITIALIZER(init_sighand.signalfd_wqh),

diff --git a/kernel/fork.c b/kernel/fork.c
index 370856d4c0b37831a3ca6e1f32b03a1124083a34..935a42d5f8ffbea46868ab9938fbf1cad0b0404a 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -710,7 +710,7 @@ static inline void free_signal_struct(struct signal_struct *sig)
 
 static inline void put_signal_struct(struct signal_struct *sig)
 {
-       if (atomic_dec_and_test(&sig->sigcnt))
+       if (refcount_dec_and_test(&sig->sigcnt))
                free_signal_struct(sig);
 }
 
@@ -1527,7 +1527,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 
        sig->nr_threads = 1;
        atomic_set(&sig->live, 1);
-       atomic_set(&sig->sigcnt, 1);
+       refcount_set(&sig->sigcnt, 1);
 
        /* list_add(thread_node, thread_head) without INIT_LIST_HEAD() */
        sig->thread_head = (struct list_head)LIST_HEAD_INIT(tsk->thread_node);
@@ -2082,7 +2082,7 @@ static __latent_entropy struct task_struct *copy_process(
                } else {
                        current->signal->nr_threads++;
                        atomic_inc(&current->signal->live);
-                       atomic_inc(&current->signal->sigcnt);
+                       refcount_inc(&current->signal->sigcnt);
                        task_join_group_stop(p);
                        list_add_tail_rcu(&p->thread_group,
                                          &p->group_leader->thread_group);