sched/core: Convert sighand_struct.count 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 sighand_struct.count 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 sighand_struct.count it might make a difference
in following places:

 - __cleanup_sighand: 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/1547814450-18902-2-git-send-email-elena.reshetova@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/fs/exec.c b/fs/exec.c
index fb72d36f7823e912716ba5ad97f5b120575bf30b..966cd98a2ce25e4eb4f72c7b089bb56aac1f77df 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1189,7 +1189,7 @@ static int de_thread(struct task_struct *tsk)
        flush_itimer_signals();
 #endif
 
-       if (atomic_read(&oldsighand->count) != 1) {
+       if (refcount_read(&oldsighand->count) != 1) {
                struct sighand_struct *newsighand;
                /*
                 * This ->sighand is shared with the CLONE_SIGHAND
@@ -1199,7 +1199,7 @@ static int de_thread(struct task_struct *tsk)
                if (!newsighand)
                        return -ENOMEM;
 
-               atomic_set(&newsighand->count, 1);
+               refcount_set(&newsighand->count, 1);
                memcpy(newsighand->action, oldsighand->action,
                       sizeof(newsighand->action));
 

diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 0b63d68dedb2a018e716aa4cb4e93663a8b46992..f912872fbf9139e06096972cdedab2ebee0d3377 100644 (file)
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -64,7 +64,7 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
        else
                bytes += kobjsize(current->files);
 
-       if (current->sighand && atomic_read(&current->sighand->count) > 1)
+       if (current->sighand && refcount_read(&current->sighand->count) > 1)
                sbytes += kobjsize(current->sighand);
        else
                bytes += kobjsize(current->sighand);

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 13789d10a50e6c2ca2b3c3792520e7ec9eab3c8e..37eeb1a28eba0f7e4d445bf18688104eadf7dad2 100644 (file)
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -8,13 +8,14 @@
 #include <linux/sched/jobctl.h>
 #include <linux/sched/task.h>
 #include <linux/cred.h>
+#include <linux/refcount.h>
 
 /*
  * Types defining task->signal and task->sighand and APIs using them:
  */
 
 struct sighand_struct {
-       atomic_t                count;
+       refcount_t              count;
        struct k_sigaction      action[_NSIG];
        spinlock_t              siglock;
        wait_queue_head_t       signalfd_wqh;

diff --git a/kernel/fork.c b/kernel/fork.c
index b69248e6f0e024c0407df16dfdc8a4919b590c78..370856d4c0b37831a3ca6e1f32b03a1124083a34 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1463,7 +1463,7 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
        struct sighand_struct *sig;
 
        if (clone_flags & CLONE_SIGHAND) {
-               atomic_inc(&current->sighand->count);
+               refcount_inc(&current->sighand->count);
                return 0;
        }
        sig = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
@@ -1471,7 +1471,7 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
        if (!sig)
                return -ENOMEM;
 
-       atomic_set(&sig->count, 1);
+       refcount_set(&sig->count, 1);
        spin_lock_irq(&current->sighand->siglock);
        memcpy(sig->action, current->sighand->action, sizeof(sig->action));
        spin_unlock_irq(&current->sighand->siglock);
@@ -1480,7 +1480,7 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
 
 void __cleanup_sighand(struct sighand_struct *sighand)
 {
-       if (atomic_dec_and_test(&sighand->count)) {
+       if (refcount_dec_and_test(&sighand->count)) {
                signalfd_cleanup(sighand);
                /*
                 * sighand_cachep is SLAB_TYPESAFE_BY_RCU so we can free it
@@ -2439,7 +2439,7 @@ static int check_unshare_flags(unsigned long unshare_flags)
                        return -EINVAL;
        }
        if (unshare_flags & (CLONE_SIGHAND | CLONE_VM)) {
-               if (atomic_read(&current->sighand->count) > 1)
+               if (refcount_read(&current->sighand->count) > 1)
                        return -EINVAL;
        }
        if (unshare_flags & CLONE_VM) {