cxl/features: bound fwctl command payload to the input buffer

In the Linux kernel, the following vulnerability has been resolved:

cxl/features: bound fwctl command payload to the input buffer

fwctl_cmd_rpc() copies cmd->in_len bytes into inbuf = kvzalloc(cmd->in_len)
and passes inbuf and in_len to ->fw_rpc(). The CXL callback cxlctl_fw_rpc()
ignores in_len and never checks the user-controlled op_size against it.

cxlctl_set_feature() bounds op_size only from below
(op_size <= sizeof(feat_in->hdr)) and then reads op_size - sizeof(hdr)
bytes from feat_in->feat_data via cxl_set_feature(). With a small in_len
and a large op_size the first memcpy() already reads past the
kvzalloc(in_len) buffer; the out-of-bounds bytes are placed in the mailbox
payload and sent to the device, and a large enough op_size can walk into
unmapped memory and oops the kernel. The Get paths pin op_size to a fixed
size but likewise read the input struct without checking in_len.

Reject, at the single dispatch point, any request whose fixed header plus
op_size does not fit in the copied-in buffer. The lower-bound test guards
the subtraction and ensures op_size was copied in before it is read.