perf/ring_buffer: Convert ring_buffer.aux_refcount 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 ring_buffer.aux_refcount 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. Please check Documentation/core-api/refcount-vs-atomic.rst
for more information.

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 ring_buffer.aux_refcount it might make a difference
in following places:

 - perf_aux_output_begin(): increment in refcount_inc_not_zero() only
   guarantees control dependency on success vs. fully ordered
   atomic counterpart
 - rb_free_aux(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and ACQUIRE ordering + 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>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: acme@kernel.org
Cc: namhyung@kernel.org
Link: https://lkml.kernel.org/r/1548678448-24458-4-git-send-email-elena.reshetova@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 284232edf9be44bb7c0c3715df1b5cdc2cf69710..5aeb4c74fb990336ea6c43b360ee074eb0befe09 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5468,7 +5468,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 
                /* this has to be the last one */
                rb_free_aux(rb);
-               WARN_ON_ONCE(atomic_read(&rb->aux_refcount));
+               WARN_ON_ONCE(refcount_read(&rb->aux_refcount));
 
                mutex_unlock(&event->mmap_mutex);
        }

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 4718de2a04e6822d98e01a63e34383ddf9a3b5b3..79c47076700af5d1276d5ccb8c63d6435fff8cf8 100644 (file)
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -49,7 +49,7 @@ struct ring_buffer {
        atomic_t                        aux_mmap_count;
        unsigned long                   aux_mmap_locked;
        void                            (*free_aux)(void *);
-       atomic_t                        aux_refcount;
+       refcount_t                      aux_refcount;
        void                            **aux_pages;
        void                            *aux_priv;
 

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 0a71d16ca41ba4e18f9c59f4e7b5dd00d418c18a..805f0423ee0b24a4ada9ec344abebf6da221c6d0 100644 (file)
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -357,7 +357,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
        if (!atomic_read(&rb->aux_mmap_count))
                goto err;
 
-       if (!atomic_inc_not_zero(&rb->aux_refcount))
+       if (!refcount_inc_not_zero(&rb->aux_refcount))
                goto err;
 
        /*
@@ -670,7 +670,7 @@ int rb_alloc_aux(struct ring_buffer *rb, struct perf_event *event,
         * we keep a refcount here to make sure either of the two can
         * reference them safely.
         */
-       atomic_set(&rb->aux_refcount, 1);
+       refcount_set(&rb->aux_refcount, 1);
 
        rb->aux_overwrite = overwrite;
        rb->aux_watermark = watermark;
@@ -689,7 +689,7 @@ out:
 
 void rb_free_aux(struct ring_buffer *rb)
 {
-       if (atomic_dec_and_test(&rb->aux_refcount))
+       if (refcount_dec_and_test(&rb->aux_refcount))
                __rb_free_aux(rb);
 }