[PATCH v4 3/3] perf/ring_buffer: Prefer struct_size over open coded arithmetic

[Erick Archer]

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "rb" variable is a pointer to "struct perf_buffer" and this
structure ends in a flexible array:

struct perf_buffer {
	[...]
	void	*data_pages[];
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + count * size" in
the kzalloc_node() functions.

In the "rb_alloc" function defined in the else branch of the macro

 #ifndef CONFIG_PERF_USE_VMALLOC

the count in the struct_size helper is the literal "1" since only one
pointer to void is allocated. Also, remove the "size" variable as it
is no longer needed.

At the same time, prepare for the coming implementation by GCC and Clang
of the __counted_by attribute. Flexible array members annotated with
__counted_by can have their accesses bounds-checked at run-time via
CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
strcpy/memcpy-family functions). In this case, it is important to note
that the logic needs a little refactoring to ensure that the "nr_pages"
member is initialized before the first access to the flex array.

In one case, it is only necessary to move the "nr_pages" assignment
before the array-writing loop while in the other case the access to the
flex array needs to be moved inside the if branch and after the
"nr_pages" assignment.

This way, the code is more safer.

This code was detected with the help of Coccinelle, and audited and
modified manually.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160 [2]
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Erick Archer <erick.archer@xxxxxxxxxxx>
---
 kernel/events/internal.h    |  2 +-
 kernel/events/ring_buffer.c | 15 ++++-----------
 2 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 5150d5f84c03..dc8d39b01adb 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -55,7 +55,7 @@ struct perf_buffer {
 	void				*aux_priv;
 
 	struct perf_event_mmap_page	*user_page;
-	void				*data_pages[];
+	void				*data_pages[] __counted_by(nr_pages);
 };
 
 extern void rb_free(struct perf_buffer *rb);
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 4013408ce012..d123fa2096cf 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -822,9 +822,7 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 	unsigned long size;
 	int i, node;
 
-	size = sizeof(struct perf_buffer);
-	size += nr_pages * sizeof(void *);
-
+	size = struct_size(rb, data_pages, nr_pages);
 	if (order_base_2(size) > PAGE_SHIFT+MAX_PAGE_ORDER)
 		goto fail;
 
@@ -833,6 +831,7 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 	if (!rb)
 		goto fail;
 
+	rb->nr_pages = nr_pages;
 	rb->user_page = perf_mmap_alloc_page(cpu);
 	if (!rb->user_page)
 		goto fail_user_page;
@@ -843,8 +842,6 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 			goto fail_data_pages;
 	}
 
-	rb->nr_pages = nr_pages;
-
 	ring_buffer_init(rb, watermark, flags);
 
 	return rb;
@@ -916,15 +913,11 @@ void rb_free(struct perf_buffer *rb)
 struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 {
 	struct perf_buffer *rb;
-	unsigned long size;
 	void *all_buf;
 	int node;
 
-	size = sizeof(struct perf_buffer);
-	size += sizeof(void *);
-
 	node = (cpu == -1) ? cpu : cpu_to_node(cpu);
-	rb = kzalloc_node(size, GFP_KERNEL, node);
+	rb = kzalloc_node(struct_size(rb, data_pages, 1), GFP_KERNEL, node);
 	if (!rb)
 		goto fail;
 
@@ -935,9 +928,9 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 		goto fail_all_buf;
 
 	rb->user_page = all_buf;
-	rb->data_pages[0] = all_buf + PAGE_SIZE;
 	if (nr_pages) {
 		rb->nr_pages = 1;
+		rb->data_pages[0] = all_buf + PAGE_SIZE;
 		rb->page_order = ilog2(nr_pages);
 	}
 
-- 
2.25.1