wifi: iwlwifi: pcie: Fix integer overflow in iwl_write_to_user_buf

wifi: iwlwifi: pcie: Fix integer overflow in iwl_write_to_user_buf

An integer overflow occurs in the iwl_write_to_user_buf() function,
which is called by the iwl_dbgfs_monitor_data_read() function.

static bool iwl_write_to_user_buf(char __user *user_buf, ssize_t count,
				  void *buf, ssize_t *size,
				  ssize_t *bytes_copied)
{
	int buf_size_left = count - *bytes_copied;

	buf_size_left = buf_size_left - (buf_size_left % sizeof(u32));
	if (*size > buf_size_left)
		*size = buf_size_left;

If the user passes a SIZE_MAX value to the "ssize_t count" parameter,
the ssize_t count parameter is assigned to "int buf_size_left".
Then compare "*size" with "buf_size_left" . Here, "buf_size_left" is a
negative number, so "*size" is assigned "buf_size_left" and goes into
the third argument of the copy_to_user function, causing a heap overflow.

This is not a security vulnerability because iwl_dbgfs_monitor_data_read()
is a debugfs operation with 0400 privileges.

Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230414130637.2d80ace81532.Iecfba549e0e0be21bbb0324675392e42e75bd5ad@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>