tty: rfcomm: prefer struct_size over open coded arithmetic

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.

This way, the code is more readable 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
Link: https://github.com/KSPP/linux/issues/160
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

diff --git a/include/net/bluetooth/rfcomm.h b/include/net/bluetooth/rfcomm.h
index 99d26879b02a53a10aee3c36d414abc0726cbeb0..c0588247690023716c8ae788bf231c1348d15c0d 100644 (file)
--- 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 69c75c041fe10aee4e11cde6e9d88cb994cd25c7..44b781e7569e3fb020c14f87bd89565042e5f968 100644 (file)
--- 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);
@@ -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;