[PATCH v2] tty: rfcomm: 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 "dl" variable is a pointer to "struct rfcomm_dev_list_req" and
this structure ends in a flexible array:

struct rfcomm_dev_list_req {
	[...]
	struct   rfcomm_dev_info dev_info[];
};

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() and copy_to_user() functions.

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 "dev_num" member is initialized before
the first access to the flex array. Specifically, add the assignment
before the list_for_each_entry() loop.

Also remove the "size" variable as it is no longer needed and refactor
the list_for_each_entry() loop to use di[n] instead of (di + n).

This way, the code is more readable, idiomatic and 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]
Signed-off-by: Erick Archer <erick.archer@xxxxxxxxxxx>
---
Changes in v2:
- Add the __counted_by() attribute (Kees Cook).
- Refactor the list_for_each_entry() loop to use di[n] instead of
  (di + n) (Kees Cook).

Previous versions:
v1 -> https://lore.kernel.org/linux-hardening/AS8PR02MB723725E0069F7AE8F64E876E8B142@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
---
 include/net/bluetooth/rfcomm.h |  2 +-
 net/bluetooth/rfcomm/tty.c     | 23 ++++++++++-------------
 2 files changed, 11 insertions(+), 14 deletions(-)

diff --git a/include/net/bluetooth/rfcomm.h b/include/net/bluetooth/rfcomm.h
index 99d26879b02a..c05882476900 100644
--- a/include/net/bluetooth/rfcomm.h
+++ b/include/net/bluetooth/rfcomm.h
@@ -355,7 +355,7 @@ struct rfcomm_dev_info {
 
 struct rfcomm_dev_list_req {
 	u16      dev_num;
-	struct   rfcomm_dev_info dev_info[];
+	struct   rfcomm_dev_info dev_info[] __counted_by(dev_num);
 };
 
 int  rfcomm_dev_ioctl(struct sock *sk, unsigned int cmd, void __user *arg);
diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
index 69c75c041fe1..af80d599c337 100644
--- a/net/bluetooth/rfcomm/tty.c
+++ b/net/bluetooth/rfcomm/tty.c
@@ -504,7 +504,7 @@ static int rfcomm_get_dev_list(void __user *arg)
 	struct rfcomm_dev *dev;
 	struct rfcomm_dev_list_req *dl;
 	struct rfcomm_dev_info *di;
-	int n = 0, size, err;
+	int n = 0, err;
 	u16 dev_num;
 
 	BT_DBG("");
@@ -515,12 +515,11 @@ static int rfcomm_get_dev_list(void __user *arg)
 	if (!dev_num || dev_num > (PAGE_SIZE * 4) / sizeof(*di))
 		return -EINVAL;
 
-	size = sizeof(*dl) + dev_num * sizeof(*di);
-
-	dl = kzalloc(size, GFP_KERNEL);
+	dl = kzalloc(struct_size(dl, dev_info, dev_num), GFP_KERNEL);
 	if (!dl)
 		return -ENOMEM;
 
+	dl->dev_num = dev_num;
 	di = dl->dev_info;
 
 	mutex_lock(&rfcomm_dev_lock);
@@ -528,12 +527,12 @@ static int rfcomm_get_dev_list(void __user *arg)
 	list_for_each_entry(dev, &rfcomm_dev_list, list) {
 		if (!tty_port_get(&dev->port))
 			continue;
-		(di + n)->id      = dev->id;
-		(di + n)->flags   = dev->flags;
-		(di + n)->state   = dev->dlc->state;
-		(di + n)->channel = dev->channel;
-		bacpy(&(di + n)->src, &dev->src);
-		bacpy(&(di + n)->dst, &dev->dst);
+		di[n].id      = dev->id;
+		di[n].flags   = dev->flags;
+		di[n].state   = dev->dlc->state;
+		di[n].channel = dev->channel;
+		bacpy(&di[n].src, &dev->src);
+		bacpy(&di[n].dst, &dev->dst);
 		tty_port_put(&dev->port);
 		if (++n >= dev_num)
 			break;
@@ -542,9 +541,7 @@ static int rfcomm_get_dev_list(void __user *arg)
 	mutex_unlock(&rfcomm_dev_lock);
 
 	dl->dev_num = n;
-	size = sizeof(*dl) + n * sizeof(*di);
-
-	err = copy_to_user(arg, dl, size);
+	err = copy_to_user(arg, dl, struct_size(dl, dev_info, n));
 	kfree(dl);
 
 	return err ? -EFAULT : 0;
-- 
2.25.1